RHI: Port post-processing effect system

A post-processing effects consists of a number of passes, each of which consists of a number of commands and a number of shaders. In the legacy GL path, this is implemented in

QSSGLayerRenderData::applyLayerPostEffects()

-> QSSGEffectSystem::renderEffect()

->  QSSGEffectSystem::doRenderEffect()

This needs to be ported/re-implemented for RHI.

Major TODOs:

• Support more than 1 effect [DONE]
• Support texture properties [DONE]
• Support more than 1 shader (i.e. both vertex & fragment) [DONE]
• Support output to & input from buffer [DONE]
• Support more than 1 pass [DONE]
• Support depth/stencil buffer [DONE]
• Optimize: fix caching of generated & compiled shaders [later]
• Fix MVP for vulkan [DONE]
• Fix buffer format & filters [DONE]
• Add lancelot test scenes [DONE]