gles: Enable the GL_EXT_read_format_bgra extension

Also fixes the usage of GL_IMPLEMENTATION_COLOR_READ_FORMAT_OES,
which may be set to a BGRA format e.g. for a MESA_FORMAT_ARGB8888 fb.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/main/APIspec.xml

@@ -3830,6 +3830,7 @@
 	<category name="OES_mapbuffer"/>
 	<category name="EXT_texture_filter_anisotropic"/>
 	<category name="EXT_texture_format_BGRA8888"/>
+	<category name="EXT_read_format_bgra"/>
 
 	<category name="ARB_texture_non_power_of_two"/>
 	<!-- disabled due to missing enums
@@ -4124,6 +4125,7 @@
 	<category name="OES_packed_depth_stencil"/>
 	<category name="OES_standard_derivatives"/>
 	<category name="EXT_texture_format_BGRA8888"/>
+	<category name="EXT_read_format_bgra"/>
 
 	<category name="EXT_texture_compression_dxt1"/>
 	<category name="EXT_blend_minmax"/>

src/mesa/main/extensions.c

@@ -198,6 +198,7 @@ static const struct extension extension_table[] = {
    { "GL_EXT_texture_env_dot3",                    o(EXT_texture_env_dot3),                    GL,             2000 },
    { "GL_EXT_texture_filter_anisotropic",          o(EXT_texture_filter_anisotropic),          GL | ES1 | ES2, 1999 },
    { "GL_EXT_texture_format_BGRA8888",             o(dummy_true),                                   ES1 | ES2, 2005 },
+   { "GL_EXT_read_format_bgra",                    o(dummy_true),                                   ES1 | ES2, 2009 },
    { "GL_EXT_texture_integer",                     o(EXT_texture_integer),                     GL,             2006 },
    { "GL_EXT_texture_lod_bias",                    o(dummy_true),                              GL | ES1,       1999 },
    { "GL_EXT_texture_mirror_clamp",                o(EXT_texture_mirror_clamp),                GL,             2004 },