meta: Disable dithering during glBlitFramebuffer

meta: Disable dithering during glBlitFramebuffer

According to the GL spec the only fragment operations that should affect
glBlitFramebuffer are “the pixel ownership test, the scissor test, and sRGB
conversion”. That implies that dithering should not be performed so we need to
disable it when implementing the blit with a render.

Before commit 05b52efbc97731 the dithering state would be left as whatever the
application picks (the default being GL_TRUE) and after that commit it was
explicitly enabled. Neither of these were correct.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=81828
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>