Resolve -Wsign-conversion warnings from clang

xcursorgen.c:169:62: warning: implicit conversion changes signedness:
  'int' to 'XcursorPixel' (aka 'unsigned int') [-Wsign-conversion]
        p       = (alpha << 24) | (red << 16) | (green << 8) | (blue << 0);
                ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
xcursorgen.c:269:39: warning: implicit conversion changes signedness:
  'png_uint_32' (aka 'unsigned int') to 'int' [-Wsign-conversion]
    image = XcursorImageCreate(width, height);
            ~~~~~~~~~~~~~~~~~~        ^~~~~~
xcursorgen.c:269:32: warning: implicit conversion changes signedness:
  'png_uint_32' (aka 'unsigned int') to 'int' [-Wsign-conversion]
    image = XcursorImageCreate(width, height);
            ~~~~~~~~~~~~~~~~~~ ^~~~~
xcursorgen.c:280:25: warning: implicit conversion changes signedness:
  'int' to 'XcursorDim' (aka 'unsigned int') [-Wsign-conversion]
    image->size = list->size;
                ~ ~~~~~~^~~~
xcursorgen.c:281:25: warning: implicit conversion changes signedness:
  'int' to 'XcursorDim' (aka 'unsigned int') [-Wsign-conversion]
    image->xhot = list->xhot;
                ~ ~~~~~~^~~~
xcursorgen.c:282:25: warning: implicit conversion changes signedness:
  'int' to 'XcursorDim' (aka 'unsigned int') [-Wsign-conversion]
    image->yhot = list->yhot;
                ~ ~~~~~~^~~~
xcursorgen.c:283:26: warning: implicit conversion changes signedness:
  'int' to 'XcursorUInt' (aka 'unsigned int') [-Wsign-conversion]
    image->delay = list->delay;
                 ~ ~~~~~~^~~~~
xcursorgen.c:364:35: warning: implicit conversion changes signedness:
  'unsigned int' to 'int' [-Wsign-conversion]
            .height             = height,
                                  ^~~~~~
xcursorgen.c:363:24: warning: implicit conversion changes signedness:
  'unsigned int' to 'int' [-Wsign-conversion]
            .width              = width,
                                  ^~~~~

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
6 jobs for misc in 1 minute and 19 seconds (queued for 2 seconds)