Mark a Clip::GetFrame() override as such

[ferdnyc]

I finally got tired of seeing this in my build output:

[104/104] Processing file .../src/effects/ObjectDetection.cpp
In file included from .../tests/FrameMapper.cpp:34:
.../src/Clip.h:233:36:
warning: 'GetFrame' overrides a member function but is not marked 'override'
[-Winconsistent-missing-override]
                std::shared_ptr<openshot::Frame>
GetFrame(std::shared_ptr<openshot::Frame> background_frame,
int64_t frame_number);
                                                 ^
.../src/ClipBase.h:104:44:
note: overridden virtual function is here
                virtual std::shared_ptr<openshot::Frame>
GetFrame(std::shared_ptr<openshot::Frame> frame, int64_t frame_number) = 0;
                                                         ^
1 warning generated.

Also, fixed some indentation while I was there.

[codecov]

Codecov Report

Merging #679 (088e318) into develop (b09416c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #679   +/-   ##
========================================
  Coverage    52.39%   52.39%           
========================================
  Files          151      151           
  Lines        12365    12365           
========================================
  Hits          6479     6479           
  Misses        5886     5886           

Impacted Files	Coverage Δ
src/Clip.h	75.00% <ø> (ø)

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