[truetype] Clean up size validation.

Rely on non-zero ppem to validate the size object.

* src/truetype/ttobjs.h (TT_Size_Merics): Remove the `valid` flag.
* src/truetype/ttobjs.c (tt_size_init,done,reset,reset_height): Updated.
* src/truetype/ttgload.c (TT_Load_Glyph): Check for non-zero ppem.
3 files changed