Transparency support

[Witek902]

Add transaprent object support to the renderer:

• transparent material
• particles
• fog / fog volumes

Think, how to connect this with deferred shading. Note that there are two types of transparency:

• additive (fire, flares, etc.)
• subtractive/multiplicative (fog, dust, etc.)

The transparent objects must be rendered in correct order (from back to front).