graphicsdriver/opengl: Fix suspicious GL function calls

Before this change, the pixel object buffer is unbound just after
getting a pointer by glMapBuffer. This seemed suspicious.

This change fixes to do all pixel manipulations once between
glMapBuffer and glUnmapBuffer without changing a bound buffer.

This might fix a wrong rendering on some machines, but I am not
sure.

Updates #993

internal/graphicsdriver/opengl/program.go

@@ -265,8 +265,6 @@ func (d *Driver) useProgram(mode driver.CompositeMode, colorM *affine.ColorM, fi
 		panic("source image is not set")
 	}
 
-	thePBOState.ensurePBOUnmapped()
-
 	if err := destination.setViewport(); err != nil {
 		return err
 	}