Handle opacity and transparent images

[henrikmidtiby]

Overview: What does this pull request change?

This PR fixes #4256

The set_opacity method now uses both the specified alpha value and the alpha channel in the original image to calculate the alpha channel of the image that is shown in the animation.

Further Information and Comments

The solution is based on PR 2923 by NicoWeio.
This PR mentions that it awaited a solution to how ImageMObject's are hashed.
This does not seem to be needed anymore.

Test code

from manim import * 

class OpacityBug(Scene):
    def construct(self):
        grid = NumberPlane(x_range=(-14, 14), 
                           x_length=7, 
                           y_range=(-10, 10), 
                           y_length=5)
        self.add(grid)

        images = [ImageMobject("pic/transparent_image.png").scale(2) for _ in range(6)]
        images[0].move_to(LEFT*2 + UP)
        images[1].set_opacity(0.80).move_to(UP)
        images[2].set_opacity(0.60).move_to(RIGHT*2 + UP)
        images[3].set_opacity(0.40).move_to(LEFT*2 + DOWN)
        images[4].set_opacity(0.20).move_to(DOWN)
        images[5].set_opacity(0.00).move_to(RIGHT * 2 + DOWN)
        self.add(images[0])
        self.add(images[1])
        self.add(images[2])
        self.add(images[3])
        self.add(images[4])
        self.add(images[5])

Output before change

Output after change

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested

[behackl]

LGTM, thank you very much!