gl_video: add RGB10_A2 FBO format

Maybe this can be the default in the future, but I'm not sure yet.

DOCS/man/en/vo.rst

@@ -422,8 +422,8 @@ Available video output drivers are:
         Selects the internal format of textures used for FBOs. The format can
         influence performance and quality of the video output. (FBOs are not
         always used, and typically only when using extended scalers.)
-        ``fmt`` can be one of: rgb, rgba, rgb8, rgb10, rgb16, rgb16f, rgb32f,
-        rgba12, rgba16, rgba16f, rgba32f.
+        ``fmt`` can be one of: rgb, rgba, rgb8, rgb10, rgb10_a2, rgb16, rgb16f,
+        rgb32f, rgba12, rgba16, rgba16f, rgba32f.
         Default: rgb.
 
     ``gamma``

video/out/gl_video.c

@@ -304,6 +304,7 @@ const struct m_sub_options gl_video_conf = {
                     {"rgba",   GL_RGBA},
                     {"rgb8",   GL_RGB8},
                     {"rgb10",  GL_RGB10},
+                    {"rgb10_a2", GL_RGB10_A2},
                     {"rgb16",  GL_RGB16},
                     {"rgb16f", GL_RGB16F},
                     {"rgb32f", GL_RGB32F},