TT_Done_Context and rename TT_Destroy_Context to TT_Done_Context with
slight changes.

* src/truetype/ttobjs.h (tt_slot_init): New function.

* src/truetype/ttobjs.c (tt_driver_init): Initialize execution context
here.
(tt_slot_init): New function to create extra points for the internal
glyph loader. We then use it directly, instead of face's glyph loader,
when loading glyph.

* src/truetype/ttdriver.c: Use tt_slot_init for glyph slot
initialization.
(TT_Load_Glyph): Load flag dependencies are handled here.
Return error if size is NULL.

* src/truetype/ttgload.c: Heavy cleanup and refactor.
(TT_Hint_Glyph): New funcion to hint a zone, prepared by caller.
(TT_Process_Simple_Glyph): Use loader->pp's instead of recalculating.
Use TT_Hint_Glyph.
No need to save/restore loader->stream before/after
TT_Vary_Get_Glyph_Deltas now.
(TT_LOADER_SET_PP): New macro to calculate and set the four phantom points.
(TT_Process_Composite_Component, TT_Process_Composite_Glyph,
load_truetype_glyph): Refactor load_truetype_glyph into these three functions.
Never set exec->glyphSize to 0. (close #13107)
Forget glyph frame before calling TT_Process_Simple_Glyph.
(TT_Load_Glyph, load_sbit_image, tt_loader_init): Refactor TT_Load_Glyph into
these three functions.
Set various fields of `glyph' here, instead of in load_truetype_glyph
and compute_glyph_metrics.