st/mesa: fix accum buffer allocation in st_renderbuffer_alloc_storage()

st/mesa: fix accum buffer allocation in st_renderbuffer_alloc_storage()

If the gallium driver doesn't support PIPE_FORMAT_R16G16B16A16_SNORM
the call to st_choose_renderbuffer_format() would fail and we'd generate
an GL_OUT_OF_MEMORY error.  We'd never get to the subsequent code that
handles software/malloc-based renderbuffers.

Add a special-case check for PIPE_FORMAT_R16G16B16A16_SNORM which is used
for software-based accum buffers.  This could be fixed in other ways but
it would be a much larger patch.  st_renderbuffer_alloc_storage() could
be reorganized in the future.

This fixes accum buffer allocation for the svga driver.

Note: This is a candidate for the 7.11 branch.

Reviewed-by: José Fonseca <jfonseca@vmware.com>