Add basic LiveTest for reading pixel rectangles.
Implemented some infracture for shader editing support

- Added new EditShaderSourceMessage message type
- Change shader tracing algorithm: do not store compilation result. Do not strace glShaderSource, store sources on pre-glDeleteShader, use stored sources only for deleted shaders. Queries for not-deleted shaders run glGetShaderSource
- Move some defines to DGLCommon/def.h
- Add UT for basic EditShaderSourceMessage usage

- Errors from EditShaderSourceMessage query are not implemented (wrong ctx, wrong shader, query failed - etc.). Maybe another message?