Add support for multiple custom post-processing effects

doc/source/eve/generic-pp-effects.rst

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+Custom Post-Processing Effects
+===============================
+
+Trinity supports having custom post-processing effects that can be applied to the rendered scene. These effects are specified in post-process volume 
+objects (see Tr2PostProcessAttributes and EveChildPostProcessVolume). The Tr2PostProcessAttributes maintains a list `genericEffects` that contains
+custom post-processing effects (of type `Tr2PPGenericEffect`). Each of these represents a custom post-processing pass.
+
+`Tr2PPGenericEffect` objects have members:
+
+- `effect`: The shader effect used for this post-processing pass.
+- `quality`: Minimal post-processing quality setting when this effect is visible.
+- `execututionSlot`: Place in the post-processing pipeline where this effect should be executed.
+- `order`: The order in which this effect should be applied relative to other effects in the same execution slot.
+
+Effect Files (Shaders)
+----------------------
+
+Effect files (.fx) for custom post-processing effects are used to define the shader programs that implement the visual effects. These effects need
+to have a `"Main"` technique defined, which serves as the entry point for the post-processing effect. Trinity will render a quad using this technique 
+to apply the effect to the scene.
+
+Trinity will pass pre-transform position (in clip space) and texture coordinates (0 to 1) to the vertex shader of the `"Main"` technique:
+
+```
+struct PostProcessVertex
+{
+    float4 pos : POSITION;
+    float2 texCoord : TEXCOORD0;
+};
+```
+
+The effect may use the the output of the previous post-processing step as the input texture. For that, the shader needs to define a texture 2D object
+with the name `"Blit"`. It is possible to have a shader that does not use the previous post-processing output, in which case the Trinity will render 
+the effect into the output of the previous pass directly. This can be used for overlay/additive effects.
+
+The effect may use other scene-wide textures, like `DepthMap` or `VelocityMap`. Note that currently, there is no guarantee that these textures will always be 
+available, especially at the execution slots later in the pipeline. Normally, `DepthMap` is safe to use at most stages, but `VelocityMap` may not be available 
+after tonemapping, or with certain rendering settings.
+
+The effect may expose parameters that can be adjusted by post-processing based on the volume intensity/relative weight. Such parameters need to have
+floating point type (e.g., `float`, `float2`, `float3`, `float4`) and have `bool IsBlendable = true;` annotation. When rendering the post-processing effect, 
+Trinity will blend these parameters based on the volume intensity/relative weight. For each effect instanced it will interpolate such parameters between their
+default values, specified in HLSL code and the values specified in the effect's parameter object.
+
+
+Blending and Grouping
+---------------------
+
+Generally, Trinity treats each custom post-processing effect as an individual pass. This means that different effects are applied separately, 
+without any automatic blending or overriding each other.
+
+For identical effects, Trinity tries to group them together to optimize rendering. Effects that share the same shader (along with all the non-blendable
+material parameters), execution slot and quality threshold are merged together, and blended as a single post-processing effect. When custom effects are 
+blended together, all of their blendable parameters are blended using the normal rules similar to any other post-processing attribute.
+
+
+Execution Slots
+---------------
+
+Execution slots determine the place in the post-processing pipeline where the effect should be executed:
+
+- `BEFORE_UPSCALING`: The effect is applied before the upscaling step or TAA in the rendering pipeline, after legacy fog and godray passes. Note, that the
+  input Src texture and the required output are in linear HDR color space. 
+- `AFTER_TONEMAP`: The effect is applied after the tonemapping step in the rendering pipeline. The input Src texture and the required output are in the 
+  final color space, typically sRGB. Note that the scene-wide textures like `DepthMap` and `VelocityMap` may not be available at this stage, and if they
+  are, they may be of a different resolution that the target one, and may have camera jitter applied.
+