Texture rectangle coordinates were clamped in the [0, dim - 1] range,
but should have been sampled in the [0.5, dim - 0.5 range] according
to the spec (a related comment was added in the code).

Also, by having getAddressingModeW() return ADDRESSING_LAYER for
rectangle textures, the 3rd texture coordinate computation will be
skipped entirely, preventing the temporary variable 'fv' from being
overwritten.

Change-Id: I4bbc30b2a2b747eae2f2a1dfb710a986bff7849b
Reviewed-on: https://swiftshader-review.googlesource.com/18768Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>