lavd/x11grab: disable drawing mouse without XFixes.

Fix a segfault if the XFixes extension is not available on
the X11 server.
Can be reproduced using the VNC server.
lavd/x11grab: change error code for unsupported visuals.
lavd/x11grab: check 32-bits color masks.

The X11 servers by VNC, at 32-bits depths, has the following masks:
R:0x000007ff G:0x003ff800 B:0xffc00000
This is not compatible with AV_PIX_FMT_0RGB32, and the result
is success with completely wrong colors.
lavd/x11grab: add an option to disable MIT-SHM.

With remote displays supporting the MIT-SHM extension,
the extension is detected and used, but attaching fails
asynchronously.
lavfi: check refcount before merging.

When merging the formats around the automatically inserted
convert filters, the refcount of the format lists can not be 0.
Coverity does not detect it, and suspects a memory leak,
because if refcount is 0 the newly allocated lists are not
stored anywhere. That gives CIDs 1224282, 1224283 and 1224284.
Lists with refcount 0 are used in can_merge_formats(), so the
asserts can not be moved inside the merge functions.
lavd/x11grab: reindent after last commit.