Add vertex arrays and buffers

Add vertex arrays and buffers
Add (some) matrix operations
Add transform operations
Texture changes:
- Throw if trying to bind a texture without a target
- Track textures per-unit
- Move the dimension getting functions from the base class to dimensioned textures
Font changes:
- Change descent of glyphs to x and y offsets
- Add wstring versions of get_string_width and draw_string
- Use a vertex array to store glyph vertices
- Add a default size for fonts