DWT-linux OpenGL Support Added

[John Reimer]

* Added OpenGL Support to DWT-Linux

In order to test the opengl snippet from the dwt-samples project, you will
need to use Derelict (installed via dsss or manually).

The GLCanvas surface allows you to use OpenGL via DerelictGL/GLU or via
standard GL import modules. However, Derelict is the preferred method of GL
support because of its simple GL version/extension loading mechanism.

Please test this feature. On my system, linux and win32 vesions of the
opengl test snippet run, but the linux version displays a filled/opaque
rotating donut while the windows version shows a wire-frame rotating donut. 
I welcome any feedback. :)

My apologies for my delay in adding this.

-JJR