glsl: avoid lowering texcoord array except in simple cases

glsl: avoid lowering texcoord array except in simple cases

With compat creeping up to geometry and tess shaders, lowering texcoord
accesses/writes becomes more complicated. Since it's an optimization
anyways, just avoid the complication for now.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>