VideoClip blitting: fix truncation bug to enable fractional opacity

[creichen]

This fixes #1369 for me, which was broken as of
fe5c782 due to a `truncate before
multiply' bug.

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it

Below is the (manual) test that I used:

from moviepy.editor import *

video = VideoFileClip('earth.mp4')

fgclip = VideoFileClip('sample-mp4-file.mp4')
fgclip = fgclip.with_end(4)
fgclip = fgclip.with_opacity(0.5)

video = CompositeVideoClip([video, fgclip])

video.subclip(0, 4).write_videofile('out.mp4', fps=5)

The second commit adds a unit test.

[coveralls]

Coverage remained the same at 68.564% when pulling 1f54b78 on creichen:fade-opacity-fix into b5e5a30 on Zulko:master.

[mondeja]

Great work, thank you for this fix @creichen! 💯