Proposal: Feature control for variable COLRv1

* include/freetype/ftdriver.h (variable-color-v1 property): Add documentation
for variable-colr-v1 property.
* src/truetype/ttdriver.c (tt_property_set): Ingest variable-control property
when called, set to enable_variable_colrv1 driver flag.
* src/truetype/ttobjs.h (TT_DriverRec): Add enable_variable_colrv1 flag.
3 files changed