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Now for every colour conversion there is a corresponding C function, allowing swapping between them for test purposes.
Here, 'ASMCOLOR' is selected and the C functions not even referenced in the makefile, see branch for use.
Permits use of USAT, for a small 1.5% speedup and smaller code.
Some other typedefs and comment shuffling copied over - should be harmless.

Version 1.50. Tagged as 'SprExtend-1_50'