[Bind] Use byte for GLboolean return types

GLboolean is typedefed as an unsigned char.  It appears that `[return:
MarshalAs(UnmanagedType.U1)]` is ignored in calli p/invokes, so we need
to take care of this ourselves by emitting byte instead of bool for the
p/invoke signature. The public API remains unchanged.

Fixes issue #129.
[Rewrite] Implement byte -> bool marshalling

Since GL_TRUE and GL_FALSE match .Net true and false (1 and 0,
respectively), we can simply reinterpret the byte value as a bool.

In the future, we could issue a `(if value == 0 then false else true)`
statement for added safety, but this does not appear to be necessary
right now.
[Bind] Fixed a System.FormatException
[ES][GL] Regenerated bindings
Merge pull request #132 from thefiddler/fixbool2

 Fix bool return types