fix: Image layer paint area fix

[ufrshubham]

Description

Fixes image layers not rendering at correct size when canvas size differs from map size.

The issue was that paint area calculations relied on _canvasSize (the viewport size), but for image layers we actually need to paint based on the map's dimensions. When someone uses a fixed-resolution camera or the viewport doesn't match the map size, this caused background images to render at the wrong size, typically showing only in the top-left corner.

The fix swaps _canvasSize for _mapSize, which is just the map's tile count multiplied by tile size. For non-repeating images, this means the paint area now correctly matches the map bounds. For repeating images, the calculation now includes both the parallax translation offset and the full map size.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• [NA] I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [NA] I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #2990

[jwest15mcarthur-web]

ITS OK

[spydon]

Lgtm!