Surface clipper cleanup.
Add EXT_gpu_shader4 support for ShaderGen2D
Begin work on ShaderGen2D vertex shaders.
Add support for ShaderGen2D shaders as valid selection.
String structure code for eventual transition from C++ to C, currently used by ShaderGen2D.

git-svn-id: https://www.williamfeely.info/svn/dxgl@449 8a90861a-4eca-46d5-b744-240ff16d0c4d