Create "Premultiplied Alpha" Node (#2578)

* Create "Premultiply Node"

This node premultiplies the RGB channels with the Alpha channel and produces an image with an "Associated pixel state". 

A lot of the graphics/workflows out there "assume" premultiplied RGBA channels so having this node available in chaiNNer's toolset will help it do more conversions that previously weren't possible in the application.

* Update premultiply.py

linting

* Delete backend/src/packages/chaiNNer_standard/image_adjustment/adjustments/premultiply.py

* Create "Premultiplied Alpha" node

This node allows the user to convert from a "Premultiplied" Alpha channel state to a "Straight/Unpremultiplied" alpha channel state, and vice versa.

* Update premultiplied_alpha.py

Try and just force dtype to float32 to clear the backend linter.

* Update premultiplied_alpha.py - Reworked

Reworked the PR to use the suggested naming and concepts.

backend/src/packages/chaiNNer_standard/image_adjustment/adjustments/premultiplied_alpha.py

@@ -0,0 +1,50 @@
+from __future__ import annotations
+
+from enum import Enum
+
+import numpy as np
+
+from nodes.properties.inputs import EnumInput, ImageInput
+from nodes.properties.outputs import ImageOutput
+
+from .. import adjustments_group
+
+
+class AlphaAssociation(Enum):
+    PREMULTIPLY_RGB = "Straight -> Premultiplied"
+    UNPREMULTIPLY_RGB = "Premultiplied -> Straight"
+
+
+@adjustments_group.register(
+    schema_id="chainner:image:premultiplied_alpha",
+    description="Converts an RGBA Input from a Straight Alpha Association to a Premultiplied Alpha Association, or vice versa.",
+    name="Premultiplied Alpha",
+    icon="CgMathDivide",
+    inputs=[
+        ImageInput(channels=[4]),
+        EnumInput(
+            AlphaAssociation,
+            label="Alpha Association Conversion",
+            default=AlphaAssociation.PREMULTIPLY_RGB,
+            option_labels={
+                AlphaAssociation.PREMULTIPLY_RGB: "Straight → Premultiplied",
+                AlphaAssociation.UNPREMULTIPLY_RGB: "Premultiplied → Straight",
+            },
+        ),
+    ],
+    outputs=[ImageOutput(image_type="Input0")],
+)
+def premultiplied_alpha_node(
+    img: np.ndarray, alpha_association: AlphaAssociation
+) -> np.ndarray:
+    rgb = img[..., :3]
+    alpha = img[..., 3]
+
+    if alpha_association == AlphaAssociation.UNPREMULTIPLY_RGB:
+        rgb_divided = rgb / alpha[..., np.newaxis]
+        return np.concatenate((rgb_divided, alpha[..., np.newaxis]), axis=-1)
+    elif alpha_association == AlphaAssociation.PREMULTIPLY_RGB:
+        rgb_multed = rgb * alpha[..., np.newaxis]
+        return np.concatenate((rgb_multed, alpha[..., np.newaxis]), axis=-1)
+    else:
+        raise ValueError(f"Invalid Alpha Association State '{alpha_association}'.")