Improvements to LCD rendering accuracy

When running the dmg-acid2 test, Peanut-GB would previously render the following screen:

Fixes to object priority and adding sprite limits on each scanline fixed the missing exclamation mark and the left mole, as shown below:

These improvements to LCD rendering are enabled by default. These can be disabled be declaring PEANUT_GB_HIGH_LCD_ACCURACY to 0 before including peanut_gb.h if these changes occur a performance penalty that is unnacceptable to your target platform.

Improvements to APU

Fixed issue #49 whereby poor APU emulation would crash Pokemon RBY. When ENABLE_SOUND was defined to 0, Peanut_GB would handle APU emulation internally. This internal APU emulation was pretty much non-existant. Minor improvements to reading and writing to APU registers fixed this issue. There is no other internal emulation on the APU.