Improved rendering performance for complex views

- vertex buffers aren't kept anymore in GPU side, instead those are now provided on every draw calls
- changed index buffer handling to D3DUSAGE_DYNAMIC from D3DUSAGE_WRITEONLY
- optimized font drawing (string length was evaluated multiple times and inside loops)
Removed extra GUIFontManager.Present() call from rendering loop. Layer manager will already call the GUIFontManager.Present().