Incorrect alpha compositing using "matplotlib.pyplot.imshow". #9906

Bug report

Bug summary

matplotlib.pyplot.imshow definition alpha compositing is incorrect, references #3343.

Code for reproduction

from matplotlib import pyplot as plt

plt.figure(figsize=(8, 8))
RGBA = plt.imread('RGB_Droplet.png')
plt.imshow(RGBA)
plt.show()

Actual outcome

Expected outcome

Composited in The Foundry Nuke:

The images used are available here:

• RGB_Droplet.png
• RGB_Droplet_Over_White_Reference.png

Matplotlib version

• Operating system: macOS
• Matplotlib version: '2.1.0'
• Matplotlib backend (print(matplotlib.get_backend())): 'module://ipykernel.pylab.backend_inline'
• Python version: '2.7.13 |Continuum Analytics, Inc.| (default, Dec 20 2016, 23:05:08) \n[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)]'
• Jupyter version (if applicable): notebook 5.0.0 py27_0