win: don't re-bind mask image when there is already one

[Monsterovich]

For some reason, it may happen that mask_image exists, causing a memory leak in win_bind_mask. Since assert(!w->mask_image); has been added, there are random crashes.

Direct leak of 56 byte(s) in 1 object(s) allocated from:
    #0 0x7fa0f26b4a37 in __interceptor_calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:154
    #1 0x555f7f92138c in default_new_backend_image ../src/backend/backend_common.c:497
    #2 0x555f7f961d4c in gl_make_mask ../src/backend/gl/gl_common.c:712
    #3 0x555f7f81f73e in win_bind_mask ../src/win.c:353
    #4 0x555f7f94ee79 in paint_all_new ../src/backend/backend.c:235
    #5 0x555f7f7f4dda in draw_callback_impl ../src/picom.c:1830
    #6 0x555f7f7f531b in draw_callback ../src/picom.c:1858
    #7 0x7fa0f3078772 in ev_invoke_pending (/lib/x86_64-linux-gnu/libev.so.4+0x5772)

It happens randomly, so this bug is hard to catch.

I just did like this: backend.c The crashes seem to be gone.

[absolutelynothelix]

it may be a good solution just to avoid crashes but we still need to figure out why exactly this happens because it may hide a bigger issue (see #1035, for example). i'd add a warning printed when refusing to bind a mask because it's already bound so this issue won't be forgotten.

and if you plan on contributing continuously, do something with these garbage merge commits.