Cast via uintptr_t when converting between integers and pointers

When casting from integer to pointer, promoting the integer to (u)intptr_t
will ensure that the resulting type can be converted to a pointer without
problems. These two casts changed in this commit trigger a warning when
building for CHERI-enabled architectures such as Arm Morello. This is not
just limited to CHERI, the cast from xcb_pixmap_t (uint32_t) to void*
should also be flagged by -Wint-to-void-pointer-cast when using Clang,
however, it appears that warning only handles C-style casts, and not
reinterpret_cast (https://github.com/llvm/llvm-project/issues/53964).