Fixes for integer cast warnings when targeting CHERI
GSIZE_TO_POINTER(): appease stricter compilers
Some compilers (e.g. CHERI LLVM) warn when casting a non-intptr_t integer type to a pointer. The GSIZE_TO_POINTER() macro thus triggers warnings which can be silence by casting to guintptr before gpointer.
NB: This macro must not be used to create valid pointers from a integer.
CHERI: silence two pedantic warnings
When building for CHERI with additional warning flags, implicitly converting uintptr_t to an integer type that can't store a pointer results in a compiler warnings. Silence two of these by adding explicit casts.