Make SpriteFont.Measure take zero allocations

[teinarss]

By using spans instead of splitting strings we avoids making allocations, which will help performance.

[teinarss]

Benchmark

|                 Method |       Mean |    Error |   StdDev |  Gen 0 | Gen 1 | Gen 2 | Allocated |
|----------------------- |-----------:|---------:|---------:|-------:|------:|------:|----------:|
|   Messure_New_one_line |   125.7 ns |  2.55 ns |  2.51 ns |      - |     - |     - |         - |
|   Messure_old_one_line |   295.0 ns |  0.66 ns |  0.59 ns | 0.0153 |     - |     - |      64 B |
| Messure_New_multi_line |   907.6 ns | 13.16 ns | 12.31 ns |      - |     - |     - |         - |
| Messure_old_multi_line | 2,163.1 ns | 30.47 ns | 27.01 ns | 0.1144 |     - |     - |     480 B |

[reaperrr]

https://github.com/OpenRA/OpenRA/wiki/Changelog-(bleed)/_compare/c8905eee60878a0f272e6fd554e216731b430076...afd4be1d00f3539e7b79f67aa2793d75e47bdeb3