access_mms: fix linkage on OS/2

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  CCLD     libaccess_mms_plugin.la
weakld: error: Unresolved symbol (UNDEF) '_vlc_poll'.
weakld: info: The symbol is referenced by:
    P:\tmp\ldconv_libaccess_mms_plugin_la-mmstu_38da54703f4316fd10.obj
Ignoring unresolved externals reported from weak prelinker.
Error! E2028: _vlc_poll is an undefined reference
file P:/tmp\ldconv_libaccess_mms_plugin_la-mmstu_38da54703f4316fd10.obj(ldconv_libaccess_mms_plugin_la-mmstu_38da54703f4316fd10.obj): undefined symbol _vlc_poll
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Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
XCB/XVideo: request correct picture format from the adaptor

This is bug compatibility for XVideo drivers that scale the whole
picture rather than the source rectangle (the XVideo 2 specification
does not really state which behaviour is correct).

This works because the video output core only applies crop parameters
*after* the vout display plugin is activated. This is brittle though:
the A/R and crop parameters should probably be passed as configuration
like zoom.
vout: document the priority rules
Wayland/SHM: fix priority
vout: update priority rules to account for SPU blending

This moves Direct3D on Windows (with no effects on actual order) and
OpenGL on Linux/BSD (now exceeding XVideo thanks shaders and blending).

It remains to be decided whether OpenGL should be increased on Windows;
in principles, it should have a higher priority than D2D and DDraw.