Initial merge of Emscripten port!

With this commit, you can compile SDL2 with Emscripten
( http://emscripten.org/ ), and make your SDL-based C/C++ program
into a web app.

This port was due to the efforts of several people, including: Charlie Birks,
Sathyanarayanan Gunasekaran, Jukka Jyl?nki, Alon Zakai, Edward Rudd,
Bruce Mitchener, and Martin Gerhardy. (Thanks, everyone!)

src/video/emscripten/SDL_emscriptenframebuffer.c

@@ -0,0 +1,136 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2014 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+#include "../../SDL_internal.h"
+
+#if SDL_VIDEO_DRIVER_EMSCRIPTEN
+
+#include "SDL_emscriptenvideo.h"
+#include "SDL_emscriptenframebuffer.h"
+
+
+int Emscripten_CreateWindowFramebuffer(_THIS, SDL_Window * window, Uint32 * format, void ** pixels, int *pitch)
+{
+    SDL_Surface *surface;
+    const Uint32 surface_format = SDL_PIXELFORMAT_BGR888;
+    int w, h;
+    int bpp;
+    Uint32 Rmask, Gmask, Bmask, Amask;
+
+    /* Free the old framebuffer surface */
+    SDL_WindowData *data = (SDL_WindowData *) window->driverdata;
+    surface = data->surface;
+    SDL_FreeSurface(surface);
+
+    /* Create a new one */
+    SDL_PixelFormatEnumToMasks(surface_format, &bpp, &Rmask, &Gmask, &Bmask, &Amask);
+    SDL_GetWindowSize(window, &w, &h);
+
+    surface = SDL_CreateRGBSurface(0, w, h, bpp, Rmask, Gmask, Bmask, Amask);
+    if (!surface) {
+        return -1;
+    }
+
+    /* Save the info and return! */
+    data->surface = surface;
+    *format = surface_format;
+    *pixels = surface->pixels;
+    *pitch = surface->pitch;
+    return 0;
+}
+
+int Emscripten_UpdateWindowFramebuffer(_THIS, SDL_Window * window, const SDL_Rect * rects, int numrects)
+{
+    static int frame_number;
+    SDL_Surface *surface;
+
+    SDL_WindowData *data = (SDL_WindowData *) window->driverdata;
+    surface = data->surface;
+    if (!surface) {
+        return SDL_SetError("Couldn't find dummy surface for window");
+    }
+
+    /* Send the data to the display */
+
+    EM_ASM_INT({
+        //TODO: don't create context every update
+        var ctx = Module['canvas'].getContext('2d');
+
+        //library_sdl.js SDL_UnlockSurface
+        var image = ctx.createImageData($0, $1);
+        var data = image.data;
+        var src = $2 >> 2;
+        var dst = 0;
+        var isScreen = true;
+        var num;
+        if (typeof CanvasPixelArray !== 'undefined' && data instanceof CanvasPixelArray) {
+            // IE10/IE11: ImageData objects are backed by the deprecated CanvasPixelArray,
+            // not UInt8ClampedArray. These don't have buffers, so we need to revert
+            // to copying a byte at a time. We do the undefined check because modern
+            // browsers do not define CanvasPixelArray anymore.
+            num = data.length;
+            while (dst < num) {
+                var val = HEAP32[src]; // This is optimized. Instead, we could do {{{ makeGetValue('buffer', 'dst', 'i32') }}};
+                data[dst  ] = val & 0xff;
+                data[dst+1] = (val >> 8) & 0xff;
+                data[dst+2] = (val >> 16) & 0xff;
+                data[dst+3] = isScreen ? 0xff : ((val >> 24) & 0xff);
+                src++;
+                dst += 4;
+            }
+        } else {
+            var data32 = new Uint32Array(data.buffer);
+            num = data32.length;
+            if (isScreen) {
+                while (dst < num) {
+                    // HEAP32[src++] is an optimization. Instead, we could do {{{ makeGetValue('buffer', 'dst', 'i32') }}};
+                    data32[dst++] = HEAP32[src++] | 0xff000000;
+                }
+            } else {
+                while (dst < num) {
+                    data32[dst++] = HEAP32[src++];
+                }
+            }
+        }
+
+        ctx.putImageData(image, 0, 0);
+        return 0;
+    }, surface->w, surface->h, surface->pixels);
+
+    /*if (SDL_getenv("SDL_VIDEO_Emscripten_SAVE_FRAMES")) {
+        char file[128];
+        SDL_snprintf(file, sizeof(file), "SDL_window%d-%8.8d.bmp",
+                     SDL_GetWindowID(window), ++frame_number);
+        SDL_SaveBMP(surface, file);
+    }*/
+    return 0;
+}
+
+void Emscripten_DestroyWindowFramebuffer(_THIS, SDL_Window * window)
+{
+    SDL_WindowData *data = (SDL_WindowData *) window->driverdata;
+
+    SDL_FreeSurface(data->surface);
+    data->surface = NULL;
+}
+
+#endif /* SDL_VIDEO_DRIVER_EMSCRIPTEN */
+
+/* vi: set ts=4 sw=4 expandtab: */