[sfnt] Apply variation deltas to PaintVarSolid.
Implement method for retrieval of an array of delta values, to be used for most of 'COLR' v1 variation implementation.
src/sfnt/ttcolr.c (FT_PaintFormat_Internal_,
get_deltas_for_var_index_base, read_paint): Add VAR_SOLID value to
enumeration, implement delta retrieval method, use delta retrieval
method for PaintVarSolid
as the first step.