misc - const, static, pass colors by reference

Make lookup tables that should not change const (so can put in flash).
Mark arguments and functions as const (protect against errors, help the compiler optimize).
Pass colors in to draw functions by reference (less copying).
Mark global variables in various files static (information hiding, may help compiler).
Error checking on bitmap example.
Make library compatible with pre C++11 syntax.
Condense code for handling more than 8 bits per color channel.