Only supported for plugins that set is_vector() to true. This returns
the frames as Gfx::VectorGraphics, which allows painting/rasterizing
them with arbitrary transforms and scales.
In order to ease the removal of this function, let's provide a no-op
default implementation so decoders can stop implementing it one by one.
First step of #19893
The ideal size is the size the user will display the image. Raster
formats should ignore this parameter, but vector formats can use
it to generate a bitmap of the ideal size.
