Integrated Graphics Flicker Fix

[andrewauclair]

This PR fixes the issues seen with integrated graphics and the docking overlays flickering like crazy. The code uses the existing transparent JPanel solution when the multi-buffer capability is available and a BufferStrategy when it is not.

A few more issues need to be fixed here:

• in full screen on Windows the dragging is laggy (maybe it's my large monitor and 3840 x 2160 resolution)
• in full screen on Linux the root docking handles are shifted to the right and bottom causing the right-side handle to be partially off screen
• the BufferStrategy solution doesn't appear to draw an overlay and handles unless you drag out of the frame and back
• the BufferStrategy solution puts the handles in the wrong place when returning from full screen