RectMask2D has a small amount of padding on the top, left and bottom edges

How to reproduce:
1. Open the attached project
2. Open SampleScene
3. Observe Game View

Expected result: You see two grey squares
Actual result: You can see two grey squares and the red background behind one of them

Reproducible with: 2019.4.12f1, 2019.4.23f1, 2020.1.0a15, 2020.3.4f1, 2021.1.3f1, 2021.2.0a13
Not reproducible with: 2018.4.33f1, 2019.4.11f1, 2020.1.0a1, 2020.1.0a14

Workaround: Use negative padding on the RectMask2D component