mesa/main: account for RTT samples when updating framebuffer

mesa/main: account for RTT samples when updating framebuffer

For EXT_multisampled_render_to_texture, we store the number of samples
in Attachment->NumSamples instead of Renderbuffer->NumSamples. This
meant that the previous code ignored that the framebuffer was
multisampled. Because of this, pipe_rasterizer_state::multisample is set
incorrectly, leading to visual artifacts on drivers that support MS-RTT
extension, such as panfrost.

Signed-off-by: Italo Nicola <italonicola@collabora.com>
Reviewed-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10747>