glsl: break the gl_FragData array into separate gl_FragData[i] variables

glsl: break the gl_FragData array into separate gl_FragData[i] variables

This avoids a defect in lower_output_reads.

The problem is lower_output_reads treats the gl_FragData array as a single
variable. It first redirects all output writes to a temporary variable (array)
and then writes the whole temporary variable to the output, generating
assignments to all elements of gl_FragData.

BTW this pass can be modified to lower all arrays, not just inputs and outputs.
The question is whether it is worth it.

Reviewed-by: Paul Berry <stereotype441@gmail.com>
v2: addressed Paul Berry's comments