[gfx] Rewrite the graphics stack to support other libraries

[usadson]

Support Direct2D gives the project a big advantage, as it isn't needed to write the graphics code first before implementing an SVG/CSS feature. After writing it using the DirectX-stack, we can then port that code over to support a wide range of devices using the wgpu stack and e.g. lyon.

This effort consists of a few different tasks:

• Port the font-specific stuff over to the main retina-gfx crate
• Move the wgpu-specific parts over to another crate from retina-gfx
• Make the retina-gfx APIs agnostic to DX/wgpu
• Initialize the retina-gfx API using DX on Windows and wgpu on other platforms

Some optional, nice-to-have's are:

• Identify resources (images, fonts, etc) by an identifier instead of a dynamic trait