input: Use cursor surface dimensions to evaluate presence of content

When setting a cursor surface, use the surface dimensions, instead of the weston_surface buffer reference, to check if the surface has any content. A weston_surface without any buffer reference may in fact have a buffer which was committed in a previous pointer entry, dropped by weston_surface and now held only internally by the renderer.

Without this fix, when a pointer enters a surface, the cursor image is not correctly updated if we set a cursor surface with an already committed buffer from a previous pointer entry, without recommitting the cursor buffer for the current entry. This can be seen, for example, in the experimental Wine Wayland driver which handles the cursor in a way that leads to the following scenario:

Setup: cursor_surface.attach(buffer) & cursor_surface.commit()
On first wl_pointer.enter: pointer.set_cursor(cursor_surface)
compositor/renderer redraws
wl_pointer.leave
On second wl_pointer.enter: pointer.set_cursor(cursor_surface)

When handling the second pointer.set_cursor() request the current code doesn't find a buffer attached to the cursor_surface (only the renderer now has a reference to it), so it doesn't update the respective view for the cursor.

The first commit is the fix itself, the last two commits implement the regression test for this issue.