Fixed rendering here and there.
The width and height of an Image are now final.
Got one step closer to implementing image rendering.
Fixed a bug involving generateImgBuffer() in ImageScreen.
RenderEngine:
* Model rendering now sets the activve GL texture to 0 if it finds an invalid texture ID.
* GLProgram unbinding now occurs after checking for a GL error.
* Fixed a potential bug involving particle rendering.
Implemented a default constructor for VertexBufferObject.