[sfnt] Improve paint limit checks

Paint tables can appear before the `base_glyphs_v1` offset if the
font is produced with the layer list before the base glyph list.  In
this case paint tables can occur after the layer list but before the
base glyph list.  Checks in the 'COLR' v1 code were rejecting fonts
with this layout.  Improve these checks by calculating a minimum
offset after which paint tables can occur and use that in safety

* src/sfnt/ttcolr.c (Colr, tt_face_load_colr): Declare
`paint_start_v1` and calculate that as the minimum of the end of
layer list and base glyph list.
(get_child_table_pointer, read_paint, tt_face_get_paint_layers):
Use that in safety checks.
2 files changed