Improve performance of content size calculation in List

[dweymouth]

Description:

Makes calculating the content size in List O(n) in the number of custom-height items rather than the number of total items. Unsurprisingly, a huge performance improvement for long lists with only a few custom item heights (as is a common use case).

Fixes #3427 (for list)

Checklist:

• Tests included. <- already covered by existing tests
• Lint and formatter run with no errors.
• Tests all pass.

Benchmark (before):

goos: darwin goarch: arm64 pkg: fyne.io/fyne/v2/widget

BenchmarkContentMinSize-8 198 6114649 ns/op 0 B/op 0 allocs/op PASS ok fyne.io/fyne/v2/widget 2.062s

Benchmark (after):

goos: darwin goarch: arm64 pkg: fyne.io/fyne/v2/widget

BenchmarkContentMinSize-8 19891040 51.04 ns/op 0 B/op 0 allocs/op PASS ok fyne.io/fyne/v2/widget 1.565s

[coveralls]

coverage: 65.069% (-0.001%) from 65.07%
when pulling b8b6b5c on dweymouth:list-size-optimization
into 54154f9 on fyne-io:develop.

[Jacalz]

Similar to #4841, I think we should add more benchmarks to List so these optimizations are easier to reason about. Apart from that, the change seems like a nice one :)

[Jacalz]

Thanks. That makes it easier to reason about