Panic if requesting a depth function and no depth buffer is available
Merge pull request #125 from tomaka/check-depth-buffer-presence

Panic if requesting a depth function and no depth buffer is available
Remove data_types.rs and clean-up vertex attributes system
Check that the attributes types are matching the ones reflected from the program
Fix doc-tests
Fix buffer not tolerant enough
Now checking attributes matching in the main thread
No longer lock the VAOs mutex while building a new VAO
Add some tests for attributes
Make "Attribute" trait unsafe
Improve panic message when attributes mis-match
Minor documentation improvement
Now panics if program attributes are missing from the vertex source
Minor internal API improvement