mesa: handle no-op cases sooner in _mesa_[Client]ActiveTexture()
mesa: handle no-op cases sooner in _mesa_[Client]ActiveTexture()

If the new texture unit is the current texture unit, we can return
before error checking.

Reviewed-by: Eric Anholt <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@anholt.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler