[truetype] Improve legibility of `glyf' parsing. * src/truetype/ttgload.c (ON_CURVE_POINT, X_SHORT_VECTOR, Y_SHORT_VECTOR, REPEAT_FLAG, X_POSITIVE, SAME_X, Y_POSITIVE, SAME_Y, OVERLAP_SIMPLE): New macros. (TT_Load_Simple_Glyph): Use new macros to make code more readable. Remove useless adjustment of `outline->tags' elements.