Huge performance impact detected with chroma key support (even though not in use...

Huge performance impact detected with chroma key support (even though not in use for a layer) in image shader. Straight alpha output support also adds a small overhead (even when not in use), so just like with blend-modes it is now conditionally compiled in the shader, and turned on in the configuration file.

New <mixer /> element added to configuration for turning on mixer features that not everybody would want to pay for (performance-wise). blend-modes also moved into this element:

<mixer>
    <blend-modes>   false [true|false]</blend-modes>
    <straight-alpha>false [true|false]</straight-alpha>
    <chroma-key>    false [true|false]</chroma-key>
</mixer>