swr: Correct texture allocation and limit max size to 2GB

swr: Correct texture allocation and limit max size to 2GB

This patch fixes piglit tex3d-maxsize by correcting 4 things:

The total_size calculation was using 32-bit math, therefore a >4GB
allocation request overflowed and was not returning false (unsupported).

Changed AlignedMalloc arguments from "unsigned int" to size_t, to handle
>4GB allocations.

Added error checking on texture allocations to fail gracefully.

Finally, temporarily decreased supported max texture size from 4GB to 2GB.
The gallivm texture-sampler needs some additional work to correctly handle
larger than 2GB textures (offsets to LLVMBuildGEP are signed).

I'm working on a follow-on patch to allow up to 4GB textures, as this is
useful in HPC visualization applications.

Fixes piglit tex3d-maxsize.

v2: Updated patch description to clarify ">4GB".

Reviewed-By: George Kyriazis <george.kyriazis@intel.com>