mesa: rework vertex shader output / fragment shader input attribute matching

mesa: rework vertex shader output / fragment shader input attribute matching

Before, if a vertex shader's outputs didn't exactly match a fragment
shader's inputs we could wind up with invalid TGSI shader declarations.
For example:

Before patch:
DCL OUT[0], POSITION
DCL OUT[1], COLOR[1]
DCL OUT[2], GENERIC[0]
DCL OUT[3], GENERIC[0] <- note duplicate [0]
DCL OUT[4], GENERIC[2]

After patch:
DCL OUT[0], POSITION
DCL OUT[1], COLOR[1]
DCL OUT[2], GENERIC[0]
DCL OUT[3], GENERIC[1]
DCL OUT[4], GENERIC[2]