meta: Abort meta pbo path if readpixels need signed-unsigned conversion

meta: Abort meta pbo path if readpixels need signed-unsigned conversion

Meta pbo path for ReadPixels rely on BlitFramebuffer which doesn't support
signed to unsigned integer conversions and vice versa.

Without this patch, piglit test fbo_integer_readpixels_sint_uint fails, when
forced to use the meta pbo path.

v2: Make need_signed_unsigned_int_conversion() a static function. (Iago)
    Bump up the comment and the commit message. (Jason)

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Iago Toral <itoral@igalia.com>