Collision editor: isometric tileset causes inconsistent transparency

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Tilesets with their grid orientation set to Isometric, with otherwise default settings, cause the collision editor to inconsistently display transparency on the editing overlays.
All editing overlays "paint" the grey background color where their transparency should be. However, when an editing overlay is large enough, and when scrolling or resizing the pane, all painted grey is cleared.
My guess is that this is a result of the default grid size (1, 1) acting as a 1 pixel grid on isometric mode (which is inconsistent with orthogonal mode) combined with a bug when using the editing tools outside the bounds of the grid.

**To Reproduce**
1. Create a new tileset
2. Set Orientation to Isometric
3. Open the collision editor
4. Attempt any editing action

**Expected behavior**
I expected the collision editor to work the same as with Orientation set to Orthogonal.

**Specifications:**
- OS: macOS 10.14.6
- Tiled Version: 1.9.2

Start by reproducing the issue in the collision editor with a new isometric tileset using the listed steps, then trace the collision-editor overlay rendering and grid-orientation handling. Compare the behavior with an orthogonal tileset while editing, scrolling, and resizing; done means transparency remains consistent in all of these cases.

Written by the indexing model from the issue text.