Add support for rendering raster images

[ivantishchenko]

Resolves #29. This extension allows us to render raster images alongside vector graphics.

This can be used to add support for rendering an <image> tag in SVG. An <image> tag contains embedded raster images. Some SVG describe vector geometric shapes on top of raster textures. https://github.com/sammycage/lunasvg/issues/113 https://github.com/sammycage/lunasvg/issues/13

Here is an example image produced with plutovg_image. There are 4 different raster textures under the vector geometric shape of a face.