Skybox implementation

Add SkyBox and CubeTexture entities, latter represents cubemap.
Add skybox shaders.
Improve texture anisotropy switching.
Improve TGAImage loading, distinguish between 32 and 24 bit images.
Fix image reloading (produced SIGSEGV).

TODO: Revove Camera <>-- SkyBox aggregation.

Issue #16.
Skybox implementation

Fix sky rendering on nvidia cards, use VBO instead of VAO to
represent skybox geometry.
Fix Mesh faces count for glDrawElements() call.
Fix dimention -> dimension misspelling.

TODO: Revove Camera <>-- SkyBox aggregation.

Issues #16, #19.
Merge pull request #16 from santa01/skybox

Implement skybox