Add support for OES_fbo_render_mipmap (#2303)

Co-authored-by: Christophe Dehais <[email protected]>

src/renderer_gl.cpp

@@ -635,6 +635,7 @@ namespace bgfx { namespace gl
 			OES_depth_texture,
 			OES_element_index_uint,
 			OES_fragment_precision_high,
+			OES_fbo_render_mipmap,
 			OES_get_program_binary,
 			OES_required_internalformat,
 			OES_packed_depth_stencil,
@@ -847,6 +848,7 @@ namespace bgfx { namespace gl
 		{ "OES_depth_texture",                        false,                             true  },
 		{ "OES_element_index_uint",                   false,                             true  },
 		{ "OES_fragment_precision_high",              false,                             true  },
+		{ "OES_fbo_render_mipmap",                    false,                             true  },
 		{ "OES_get_program_binary",                   false,                             true  },
 		{ "OES_required_internalformat",              false,                             true  },
 		{ "OES_packed_depth_stencil",                 false,                             true  },