gdi32: use HandleToULong inline function to convert handle to index instead of casting

[Daniel Lehman]

> It can't, the code explicitly casts handle to unsigned before the
> shift.

It's cast to a 64-bit unsigned, so the upper 32 bits are kept for the shift

One of the pointers passed in was: 0xffffff000026a8  

after the shift, it became:
0xffffff000026a8 => 0x3fffffc00009aa

When assigned to the 32-bit i, the upper 32 bits were clipped out:
0x3fffffc00009aa => 0xc00009aa

The HandleToUlong() has an additional cast to ULONG32 that drops the upper 32 bits before the shift.  Alternatively, I could have just added the cast but I noticed that macro was used elsewhere when converting handles

I'll admit that the upper 32 bits being set was because of a quirk in our C++ code, but similar values passed to Windows worked

Thanks
daniel