with-gl-context in canvas%

Calls a thunk with the canvas as the OpenGL context. The thunk typically calls OpenGL routines to draw to the canvas. MrEd, however, provides no OpenGL routines; an extension to MrEdmust be used instead. The result of the method call is the result of the thunk.

• (send a-canvas with-gl-context thunk) -> return value of thunk
   thunk : procedure of no arguments