Remove hard-coded shaders path.

Made it relative to the project location, specified in the .pro file.
Fix orbit camera.

- calculate proper modelview/projection matrices.
- pass in near/far clip planes to shader. Don't use gl_DepthRange.
- remove need for texture coordinates to map fragments. Use gl_FragCoord.
Implement first "solid" voxel mode based in DDA traversing.

Some basic coloring in a dense grid, doesn't scale very well, but the
traversal is trivial and we can modify the data as much as we want.

There's quite a few warnings now I bumped the GLSL version to 130 because
of glTextureFetch -- I need to remove the now deprecated stuff in the
shader.
Merge branch 'master' of https://github.com/joesfer/voxelToy