Clip: Use QPainter opacity

[ferdnyc]

Hey, I loves me a good bit-banging as much as the next guy.

(OK, that's a lie, I completely hate it and feel like, if we're fiddling with the raw, individual bits of an image, we're way too low-level and should be using better APIs or support classes.)

But I especially can't countenance openshot::Clip() manually applying alpha transparency in a for() loop, immediately before it uses QPainter to draw the resulting image... when QPainter has a .setOpacity() method!!

...So let's just use that, why don't we?

Edit: Also, I finally figured out what that one Codacy message was complaining about, with the source_image in Clip::apply_keyframes. (We were double-encapsulating the waveform QImage in a std::shared_ptr.) Fixed.

[codecov]

Codecov Report

Merging #637 (d91a8a3) into develop (0d522e1) will increase coverage by 0.04%.
The diff coverage is 70.00%.

@@             Coverage Diff             @@
##           develop     #637      +/-   ##
===========================================
+ Coverage    51.85%   51.90%   +0.04%     
===========================================
  Files          151      151              
  Lines        12332    12329       -3     
===========================================
+ Hits          6395     6399       +4     
+ Misses        5937     5930       -7     

Impacted Files	Coverage Δ
src/Clip.cpp	44.52% <57.14%> (+0.63%)	⬆️
src/QtImageReader.cpp	74.19% <100.00%> (+0.42%)	⬆️

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[github-actions]

Merge conflicts have been detected on this PR, please resolve.

[ferdnyc]

This code has changed so much in the wake of #638 that I can't even begin to reconcile it.