Added hardware accelerated support for palettized textures

Supported backends: direct3d, direct3d11, direct3d12, gpu, metal, opengl, opengles2

src/render/opengl/SDL_shaders_gl.h

@@ -31,6 +31,8 @@ typedef enum
     SHADER_INVALID = -1,
     SHADER_NONE,
     SHADER_SOLID,
+    SHADER_PALETTE,
+    SHADER_PALETTE_PIXELART,
     SHADER_RGB,
     SHADER_RGBA,
     SHADER_RGB_PIXELART,