shader: on GLES replace FS COLOR inputs that are not availabe by constants

Legacy FS shaders may use glColor and glSecondaryColor without the VS actually emitting it. However, on GLES the interfaces between stages must be matched, so replace FS input colors with a constant when they are not emitted by the VS.

Signed-off-by: Gert Wollny gert.wollny@collabora.com