Fix LaserZap LineRenderer flushing.
Clean up LineRenderer whitespace (no code changes).
Move IsPowerOf2 to Exts.
Generalize SheetBuilder overflow behavior.
Expose FBOs to engine code.
Texture changes:

* The GL texture id is now readonly.
* Added Size property.
* Added GetData() for reading data back from the GPU.
* Added SetEmpty() for creating an empty texture of a given size.
Allow sheets to wrap an ITexture directly.
Extract voxel transform matrix into a function.
Add functions for calculating voxel bounding boxes.
Move MakeFloatMatrix to Graphics.Util.
Add a visualization layer for renderable geometry.
Support sprites with an internal offset.
Use PaletteReferences everywhere.
Add WorldRgbaSpriteRenderer.
Remove nearest-int position rounding from SpriteRenderer.

The things that want to be rounded already do this
much earlier (wr.ScreenPxPosition, etc).
Introduce Renderable.BeforeRender().

WorldRenderer.Draw() has been slightly reorganized
to ensure that BeforeRender is called before any
drawing render state (e.g. scissor) has been set.
Support rendering sprites into non-rectangular quads.
Reimplement voxel rendering with a FBO.
Remove obsolete Stencil Buffer functions.
Allow/require renderers to enable alpha blending when needed. Fixes voxel shadow issues.