Finally make the bloody blur effect work properly.

This fixes the artefacts appearing when only part of the screen is updated.

This version also brings ton of optimizations which might well increase performance 2 or 3 times on slower cards:
- Windows are not drawn twice anymore. Now they're drawn only to render target and later changed parts of the render target are copied back onto screen.
- Shaders have been optimized. Some calculations moved from pixel shader to vertex shader.
- For ARGB windows, if window's opacity is 0 then it will stay transparent instead of being replaced by blurred background.
- Blur effect should now play nicer with other effects, e.g. shadows.