[sfnt] Apply variation deltas to `PaintVarSolid`.

* src/sfnt/ttcolr.c (FT_PaintFormat_Internal_): New enumeration
`FT_COLR_PAINTFORMAT_INTERNAL_VAR_SOLID`.

(get_deltas_for_var_index_base) [TT_CONFIG_OPTION_GX_VAR_SUPPORT]: New
function to retrieve an array of delta values, which will be used for most
of 'COLR' v1 variation formats (in follow-up commits).

(read_paint): Add `face` parameter; update caller.
Handle `FT_COLR_PAINTFORMAT_INTERNAL_VAR_SOLID`.
1 file changed