glsl: report correct number of allowed vertex inputs and fragment outputs

glsl: report correct number of allowed vertex inputs and fragment outputs

Before we would always report 16 for both and we would only fail if either
one exceeded 16. Now we fail if the maximum for each is exceeded, even if
it is smaller than 16 and we report the correct maximum.

Also, expand the size of to_assign[] to 32. There is code at the top
of the function handling max_index up to 32, so this just makes the
code more consistent.

Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>