Fix scaling functions

[f-fl0]

Description

This PR corrects two small errors that causes the improper results reported in the referenced issue:

• A copy paste typo (most likely) which sets the Dy component of the image rectangle to Dx.
• The tmp array in fastBoxSampling was not properly reset because the last argument in memset should be the number of bytes to fill in and not the number of elements.

Reference issue

Fixes https://github.com/pion/mediadevices/issues/312

[codecov[bot]]

Codecov Report

Merging #385 (466d14b) into master (1f92ea4) will increase coverage by 0.74%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #385      +/-   ##
==========================================
+ Coverage   46.42%   47.16%   +0.74%     
==========================================
  Files          67       67              
  Lines        4373     4380       +7     
==========================================
+ Hits         2030     2066      +36     
+ Misses       2220     2189      -31     
- Partials      123      125       +2     

Impacted Files	Coverage Δ
pkg/io/video/scale.go	66.12% <100.00%> (ø)
pkg/io/video/scaleycrcb.go	92.30% <100.00%> (+2.83%)	:arrow_up:
pkg/io/video/scaler_cgo.go	60.55% <0.00%> (+26.60%)	:arrow_up:

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[f-fl0]

I found another problem when using the nearest neighbor scaling function when the same size as the input image:

It is demonstrated by the test added in d56d4fb42127acf4b34b821c81aca55dc453f99b. It is fixed by 89d120e98551fb54d218c8d1935910ef69e6b921.

[f-fl0]

The need for the fix introduced in ee67dee4b812bb68e026cfb0b57115d9fb8a1b45 is demonstrated by the test added in 5f2216ace1313f2e521e9fe4b5ee0b82bff0f590.

[f-fl0]

The need for the fix introduced in ad6b0ad5e1c7878befe0dca92d82e09b3dae345d is demonstrated by the test added in edef5f5dee11be34eb8c12b719064b2764b6fd4d

[f-fl0]

@at-wat I added some tests and tried to demonstrate they would fail on master by reverting each change and let CI fail.