[truetype] Rework handling of rendering mode in bytecode. This replaces a large number of confusing boolean constructs that describe the target rendering mode in the execution context with the straight mode variable. It might fix some hidden bugs when FT_LOAD_TARGET_XXX were used as flags, which they are not. The condition that triggers the CV program re-execution is simplified. These events due the rendering mode change are rather rare and unexpected and, therefore, should not be over-analyzed. In v40, all mode changes now trigger the CV program. In v35, only switches to/from mono do. * src/truetype/ttinterp.h (TT_ExecContext): Replace 'grayscale', 'grayscale_cleartype', 'subpixel_hinting_lean', and 'vertical_lcd_lean' with the rendering 'mode'. * src/truetype/ttinterp.c (Ins_GETINFO): Updated. * src/truetype/ttgload.c (tt_loader_init): Replace 'prep' re-execution trigger and update 'backward_compatibility' condition. (tt_loader_set_pp): Updated.