Switch draw_test_pattern() to call libpixpat (linked statically into
libkms++util.so) instead of the in-tree pattern generator. Pixpat
covers every pattern (kmstest, smpte, solid colors) and every pixel
format the previous generator handled, so behavior is unchanged for
all callers.
Drop the now-unused machinery: conv.h and conv-*.h template writers,
color16.h (RGB16/YUV16 plus conversions), the *_old / _single / _multi
declarations that had no definitions or callers, and the c_draw_test_pattern
C ABI which had no callers anywhere in the tree. RecStandard and
ColorRange move from color16.h directly into kms++util.h, since they
are still part of the public TestPatternOptions struct.