vrend: copy_fallback: limit per slice transfer to slice size

When calling glGetnTexImage, glGetnCompressedTexImage, or glCompressedTexSubImage*D and the data pointer is updated with the per-slice offset but as bufSize value the full length of the buffer was given, which can result in a crash of qemu when the buffer size was not evaluated correctly. Instead of passing the size of the whole buffer pass the size of the slice to make sure that glGetnTexImage doesn't write to unallocated memory. Closes: #1 (closed)

Signed-off-by: Gert Wollny gert.wollny@collabora.com