Interpolated getter analogous to the setter and a separate, dimension agnostic bound checking for both.

mainly splitting it into .h,.hpp,.cpp, providing template instantiation
for all available pixel formats (4 now, more in the future).
Also some small improvements and fixes that I wasn't able to keep track
of because of the restructure -_-
Why can't I do things one at a time? >.<

for when SDL doesn't provide those.

Also rgb888 is actually xrgb8888??? weird... will need to provided
actual rgb888

and a couple of minor const noexcept changes.

as in N^2 long...