[truetype/GX] Clean up advance adjustment.
- src/truetype/ttgload.c (TT_Process_Simple_Glyph, load_truetype_glyph): Move the advance adjustment from here...
- src/truetype/ttgxvar.c (TT_Vary_Apply_Glyph_Deltas): ... to here.
- src/truetype/ttgxvar.h (TT_Vary_Apply_Glyph_Deltas): Update prototype.