gr_init_bitmap can be passed a pointer to uninitialized memory, and
will save that pointer into the bitmap for the underlying memory to be
initialized later. In gcc-11, this triggers a warning because the
`const` qualifier leads gcc to expect that the memory will only be read
(while uninitialized) and not written.
Reported-by: TheDemonicSurfer <https://github.com/dxx-rebirth/dxx-rebirth/issues/593>
32b7899059