Alternate render targets and dynamic textures

Currently the engine can only render to a window. Need to allow other targets in order to facilitate user interface development, compositing, etc.

Primary cases:

Generate a texture using C# code and use it as a texture resource
Make GPU draw calls to a texture render target and then use that as a texture resource
Support 1D, 2D, and 3D textures, as well as texture cubes and texture arrays
[x] Create and update texture resources by drawing directly to texture memory on the CPU side
[ ] Render to a texture resource (render target) using GPU draw calls
[ ] Merge multiple dynamic texture resources into a final composite scene, with alpha blending

axefrog / Grasshopper