Instead of testing for the number of samplers used so far
in the shader, the Program object was checking for the
register index, which could have been larger or equal to
MAX_VERTEX_TEXTURE_IMAGE_UNITS or MAX_TEXTURE_IMAGE_UNITS,
making the shader compilation fail erroneously.

Changed sampler arrays to maps in order to get the sampler
data from the register index.

Change-Id: Iba6cbf0dbbcdbd926f2670af4413710550e341a9
Reviewed-on: https://swiftshader-review.googlesource.com/15428Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>