Fix weird MSVC compilation error

[Gaktan]

For some reason, MSVC doesn't appreciate uint8 math with enum constants.
I have no idea why I didn't have this error until a few weeks ago. The incriminating code was has not been changed recently which is super weird.
I still had the error even after updating VS2019.

The error is caused by the use of << FRAC_BITS inside the if constexpr (sizeof(T) == 1) { condition in _scale_bilinear.
I tried a few different things, but declaring FRAC_BITS as uint32 seems to be the only thing that works.
It doesn't hurt to clean the code though, using an enum to store constants is error prone, since the type is inferred by the compiler.

Error log:

1>core\io\image.cpp(4325): fatal error C1001: Internal compiler error.
1>(compiler file 'D:\a\_work\1\s\src\vctools\Compiler\CxxFE\sl\p1\c\PackExpander.cpp', line 2441)
1> To work around this problem, try simplifying or changing the program near the locations listed above.
1>If possible please provide a repro here: https://developercommunity.visualstudio.com
1>Please choose the Technical Support command on the Visual C++
1> Help menu, or open the Technical Support help file for more information
1>core\io\image.cpp(1272): note: see reference to function template instantiation 'void _scale_bilinear<1,uint8_t>(const uint8_t *__restrict ,uint8_t *__restrict ,uint32_t,uint32_t,uint32_t,uint32_t)' being compiled

[akien-mga]

This should be reported to Microsoft as a bug in MSVC, since it's an internal compiler error on valid code.

The proposed workaround might be fine to merge to solve it downstream but ideally the bug should be fixed in the compiler, so we don't run into it in other cases.

[akien-mga]

Thanks!