trim: Trim most drawing operations outside of the user-specified range

trim: Trim most drawing operations outside of the user-specified range

Care is taken to consider a glBegin/glEnd block as a single drawing
operation, (so that it will enitrely included or entirely trimmed).

We're also careful to avoid trimming any drawing operation when there
are active side effects (other than rendering to the default
framebuffer). For example, if there is an active binding of
GL_FRAMEBUFFER or GL_DRAW_FRAMEBUFFER to any buffer object, or if we
are withing a glBeginTransformFeedback/glEndTransformFeedback block;
in any of these cases the drawing operation will not be trimmed.