Call structure for some GLX and OpenGL calls

OpenGLThread::handleFifo

OpenGLThread::loadTEX
- loadYUVTEX
  - glBindBufferARB, glBindTexture, glTexSubImage2D
OpenGLThread::render (SlotNumber n_slot)
- OpenGLThread::render_lists = list of RenderGroup (s) per slot
  - RenderGroup::render glXMakeCurrent, XGetWindowAttributes, glViewport, glClear
    - RenderGroup::render_contexes = list of RenderContext (s)
      - RenderContext::render
        
        Shader::use
        
        OpenGLThread::bindTextures()
        
        glActiveTexture, glBindTexture, glUniform1i
        
        OpenGLThread::bindVars()
        
        glUniformMatrix4fv
        
        OpenGLThread::bindVertexArray()
        
        glBindVertexArray, glDrawElements, glBindVertexArray
    - glXSwapBuffers

SlotContext manages textures:

Several RenderContext instances (i.e. mappings) can refer to the same SlotContext.

This way we can map the same texture into several x-windows and also into "picture-in-picture" schemes. Let's state this idea graphically:

* 
* 
* 
*                                        (RenderGroup 1 = x window 1)
*                                        +------------+
*                                        |+-----+     |
*                                        ||     |     |  ("map slot 1 to RenderGroup 1")
*                  +--- RenderContext---->|     |     |
*                  |                     |+-----+     |
* SlotContext -----+                     +------------+
*                  |                     
*                  |                     +------------+
*                  +-- RenderContext---->|            |  ("map slot 1 to RenderGroup 2")
*                                        |            |
*                                        |            |
*                                        |            |
*                                        +------------+
*                                        (RenderGroup 2 = x window 2) 
* 
*