Glyph paint callbacks.

The callbacks assume that the caller maintains a stack of current transforms, clips and intermediate surfaces, as evidenced by the pairs of push/pop callbacks. The push/pop calls will be properly nested, so it is fine to store the different kinds of object on a single stack.

Not all callbacks are required for all kinds of glyphs. For rendering COLRv0 or non-color outline glyphs, the gradient callbacks are not needed, and the composite callback only needs to handle simple alpha compositing (HB_PAINT_COMPOSITE_MODE_SRC_OVER).

The paint-image callback is only needed for glyphs with image blobs in the CBDT, sbix or SVG tables.

The custom-palette-color callback is only necessary if you want to override colors from the font palette with custom colors.

Since: 7.0.0