Performance optimizations for the GBC code 🐢

Description

Now that the CGB code has been merged !16 (merged) we need to focus on the optimization process to be able to have CGB support without to much of a performance impact.

Analyze the performance impact of this commit f61e3f9e because of the whole let cycles_n = cycles / self.mmu().speed.multiplier();.

The main areas that should be the focus of optimizations are:

• The PPU conditional code for CGB and DMG with color compatibility
• The variable clock speed code of the main GB cycle https://gitlab.stage.hive.pt/joamag/boytacean/-/blob/master/src/gb.rs#L396
• The usage of the get_flipped() method seems to be one of the culprits

⚠️ The performance impact of the CGB changes is measured to be in the order of 30% less performance. And most of it should be recoverable.

Version 0.7.4 of Boytacean should be used as a reference for benchmarking.