Drawing with null CURRENT_PROGRAM should error.

This is actually Chrome's behavior already. Firefox ignores it, in line with the GLES2 spec.
Merge branch 'draw-null-program' of github.com:jdashg/WebGL into draw-null-program
Add a conformance test for null-program draws.
Merge pull request #510 from jdashg/draw-null-program

Drawing with null CURRENT_PROGRAM should error.