egl: Implement getImage/putImage on pbuffer swrast.

egl: Implement getImage/putImage on pbuffer swrast.

This change adds getImage/putImage callbacks to the swrast pbuffer
loader extension.
This fixes a recent crash with Weston as well as a crashing
test with classic swrast without an official gitlab issue.

v2: Determine bytes per pixel differently and fix non X11 builds.
v3: Plug memory leak and fix crash on out of bounds access.
    (Daniel Stone)
v4: Follow the code structure of the wayland get/put image
    implementation - hopefully being more obvious.
    Handle 64 bits formats.
    Use BufferSize directly.
    (Emil Velikov)
v5: Change pixel size computation.
    (Eric Engestrom)

Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2219
Fixes: d6edccee8da "egl: add EGL_platform_device support"
Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3711>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3711>