ui: Move Font to ui namespace, improve ui::Renderer

Renderer now resizes the vertex buffer automatically and provides basic (stubbed) interface for 2D drawing
gfx: Fix Format_ShortN to use signed shorts
ui: Implement Renderer::text

Uses simple kerning-aware single-line layout; currently vertical layout always uses ascender for positioning
ui: Renderer now supports high-density font rendering

Layout is performed in pixel units, not in canvas units.