Fix linestrip usage for highlight box
Implement glMemoryBarrier
Always point sample depth textures on display
Fix detecting SInt textures from SNorm accidentally
A bit of a hack, only show stencil when exclusively selected

* Thumbnails enable all channels, so if depth (Red) is enabled then ignore
  that stencil (Green) is also enabled.
Implement min/max calculation for OpenGL

* Same implementation as D3D11 essentially
* Split out the texture sampling from texdisplay.frag into its own file so
  it can be reused. Also removed the discard;s from the texture sample
  functions and moved them up into the texture display function.
Implement glClearBufferData
Implement texture histogram for OpenGL