LibGfx: Return BPP for all supported image formats

[shlyakpavel]

This commit moves some of the actual code from the header file to the CPP file to reduce header size and improve encapsulation. Additionally, it ensures that the bpp is correctly returned for all supported image formats, which may help prevent potential runtime crashes in the future.

[konradekk]

this was neat ersatz for pattern matching, probably worth keeping in the new location.

[shlyakpavel]

The current code ensures that all values are validated at compile-time, which provides a significant advantage: it prevents the possibility of introducing a new color space without explicitly adding support for it in this function. This approach eliminates the risk of runtime crashes due to unhandled cases, as any missing implementation will result in a compile-time error rather than a runtime failure.

[ADKaster]

Does any caller call this method with untrusted user input? Could any caller be reasonably created to call with untrusted user input? And by untrusted user input I am including image data from an undecided image, as well as information that comes from IPC.

[gmta]

In this case, since it returns, the VERIFY_NOT_REACHED() still should be added - just after the switch.