Issues with WEBGL_debug extension spec

[kenrussell]

A couple of issues with this extension specification came up during a recent face-to-face meeting:

• The MAX_DEBUG_MESSAGE_LENGTH_KHR and MAX_DEBUG_GROUP_STACK_DEPTH_KHR enums should be removed since they're unnecessary (the extension just returns strings to JavaScript).
• The extension should mention that DEBUG_OUTPUT_SYNCHRONOUS was removed. This eliminates undefined behavior from reentrant debug callbacks.

Also, hopefully there won't be issues with needing programs' line numbers to be the first thing on the line, etc. (as with getProgramInfoLog in the past).