Added glIs*

[jeaye]

They're currently returning GLboolean, which is actually a u8. This is how the ll bindings expose them, so I kept it that way, but it'd be nice to just have them return Rust booleans. The current syntax looks something like:

if gl::is_buffer(name) > 0
{ ... }

and it'd just flow better as:

if gl::is_buffer(name)
{ ... }

Let me know what you think.

Cheers,
Jeaye

[metajack]

A quick pick through the other functions shows that they take bools, not GLboolean. I think we should return bools as well. I can see no reason not to do so.

Otherwise, this all looks good.

[jeaye]

Done, thanks.