Clean up shaders.

[dragostis]

Currently, some shaders use templating, others don't, while also using different invocation styles.

We should:

• organize shaders better with the use of imports
• have more variables be configurable by templating (e.g. workgroup size)
• use the same value for max invocations per dispatch and have one single method to deal with this limitation.

[ayshiff]

I would be happy to work on that! 👍

[dragostis]

Let's start by finding the right template engine to use. Ideally, a very small one with import support would be ideal. Currently, we're using ramhorns, but maybe there's a better alternative?

Also, maybe there's an alternative that can do most of the work offline (i.e. at compile time) and leave perhaps one or tweo variables overiddable.