Amalgamation of the WebGL constants from the IDL interfaces in
WebGLRenderingContextBase, WebGL2RenderingContextBase, & WebGLDrawBuffers.
Because the RenderingContextBase interfaces are hidden they would be
replicated in more than one class (e.g., RenderingContext and
RenderingContext2) to prevent that duplication these 600+ constants are
defined in one abstract class (WebGL).