mesa/st: Add checks for signed/unsigned integer conversions in ReadPixels

mesa/st: Add checks for signed/unsigned integer conversions in ReadPixels

These checks were in Mesa prior to commit fbba25bba, but they were
not necessary for the purpose that Mesa intended (check if we could
resolve ReadPixels via memcpy), so that commit took them away.

Unfortunately, it seems that some Gallium drivers rely on these
checks to make the decision of whether they should fallback to Mesa's
implementation of ReadPixels correctly. Michel Dänzer reported that
the following piglit test would fail on radeonsi after commit
fbba25bba:

spec@ext_texture_integer@fbo_integer_readpixels_sint_uint

This patch puts the checks back in Gallium, where they are needed.

Tested-by: Michel Dänzer <michel.daenzer@amd.com>