cursor: Fix heap overflows when parsing malicious files.

It is possible to trigger heap overflows due to an integer overflow
while parsing images.

The integer overflow occurs because the chosen limit 0x10000 for
dimensions is too large for 32 bit systems, because each pixel takes
4 bytes. Properly chosen values allow an overflow which in turn will
lead to less allocated memory than needed for subsequent reads.

See also: https://cgit.freedesktop.org/xorg/lib/libXcursor/commit/?id=4794b5dd34688158fb51a2943032569d3780c4b8
Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=103961



Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
[Pekka: add link to the corresponding libXcursor commit]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>