core: Convert_Scale: Add support for > 8bits.

Add support for conversion macros up to 32bits
per channel and also make the header more readable
by writing the shift as a C shift.
loaders: PSP: Fix 'set but not used' warnings.
pylib:templates,gfxprim_config: Add 16bit Grayscale.

Add 16 bit grayscale into the config,
fix filter templates.
pywrap:core: Make members of GP_DebugMsg immutable.