add doc/*/libgfx-posteffects.md

doc/en/scripting/builtins/libgfx-posteffects.md

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+# gfx.posteffects library
+
+A library for post-processing effects control.
+
+The effect slot is a resource and must be declared in resources.json in the root directory of the pack:
+
+```json
+{
+    "post-effect-slot": [
+        "slot_name"
+    ]
+}
+```
+
+```lua
+-- Returns the index of the effect slot by name (pack:slot_name).
+-- If the specified slot does not exist, returns -1
+gfx.posteffect.index(name: str) --> int
+
+-- Assigns the effect to the slot
+gfx.posteffect.set(slot: int, effect: str)
+
+-- Returns the effect intensity (from 0.0 to 1.0)
+-- If the slot is empty, returns 0.0
+gfx.posteffect.get_intensity(slot: int) --> number
+
+-- Sets the effect intensity (from 0.0 to 1.0)
+-- (The correctness of processing the parameter between 0.0 and 1.0 
+-- depends on the effect)
+gfx.posteffect.set_intensity(slot: int, intensity: number)
+
+-- Returns true if the slot is not empty and the effect intensity is non-zero
+gfx.posteffect.is_active(slot: int) --> bool
+
+-- Sets parameters values ('param' directives)
+gfx.posteffect.set_params(params: table)
+```