Updated SDL's YUV support, many thanks to Adrien Descamps

New functions get and set the YUV colorspace conversion mode:
	SDL_SetYUVConversionMode()
	SDL_GetYUVConversionMode()
	SDL_GetYUVConversionModeForResolution()

SDL_ConvertPixels() converts between all supported RGB and YUV formats, with SSE acceleration for converting from planar YUV formats (YV12, NV12, etc) to common RGB/RGBA formats.

Added a new test program, testyuv, to verify correctness and speed of YUV conversion functionality.

src/render/opengl/SDL_shaders_gl.h

@@ -26,9 +26,15 @@ typedef enum {
     SHADER_NONE,
     SHADER_SOLID,
     SHADER_RGB,
-    SHADER_YUV,
-    SHADER_NV12,
-    SHADER_NV21,
+    SHADER_YUV_JPEG,
+    SHADER_YUV_BT601,
+    SHADER_YUV_BT709,
+    SHADER_NV12_JPEG,
+    SHADER_NV12_BT601,
+    SHADER_NV12_BT709,
+    SHADER_NV21_JPEG,
+    SHADER_NV21_BT601,
+    SHADER_NV21_BT709,
     NUM_SHADERS
 } GL_Shader;