mesa: Pure software accum buffer.

mesa: Pure software accum buffer.

The existing implementation was already implemented on software, but relied
on the pipe driver to always support the R16G16B16A16_SNORM format. This
patch eliminates that, without prejudice against a future hardware-only
implementation.

It also avoids some of the short <-> float conversions, and only does a read
transfer of the color buffer on GL_RETURN if absolutely necessary.