model viewer improvement spike. multiple changes.

-Drop to .net 4.5.2, show console for now, linear texture sample.
-Cleanup logic for computing depth bias
-Increase depth bias to decrease z-fighting
-Optimize async texture loading at map-load time.
-Create solid color textures in-memory rather than via resource.
-Revert to point filtering as linear led to transparent edges.
-Make unloaded texture less of an eyesore
-Remove unused LoadMeshThreadArg
-Draw wires around meshes that share the same texture
-Use WPF to handle message pump.
-Only render when scene has been updated.