bugifix:
- implement sdl2rendercontext::activate.
- add return values for render context.

Signed-off-by: Kim Kulling <kim.kulling@googlemail.com>
refactoring:
- remove not used dependencies from rendersystem build.
- remove deprecated implementation of OGLRenderContext.

Signed-off-by: Kim Kulling <kim.kulling@googlemail.com>