Dashes in a line are drawn at a minimum 0.05px before scaling

* .NET Core Version: 3.1
* Windows version: Windows 10, Version 1909
* Does the bug reproduce also in WPF for .NET Framework 4.8?: *Partially* (some variations on the bug introduced in Core; see below)
* Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? No

**Problem description:**

There are a handful bugs related to dashes. The first one is why I wrote this report, but the other two came up as I tried to create a minimal repro.

1. When a dashed line is drawn in a `DrawingContext` such that *before scaling* the length of the dash is *shorter* than **0.05px**, then the dash is rendered at 0.05px *before scaling*.
2. The final dash in a line may either not be drawn (.NET Framework 4.8) or blink in and out while scrolling horizontally (.NET Core 3.1).
3. The last dash in a line may be longer than the other ones.

**Actual behavior:**

1. When very small dashes are scaled up, they are larger than they should be. e.g. A 0.01px dash before scaling, scaled 100x, should be 1px long. However, it is first rendered at the minimum 0.05px, then scaled 100x to 5px.
2. See above.
3. See above.

**Expected behavior:**

1. Dashes should not have a size cap before scaling.
2. Lines aren't missing any dashes, and they are drawn regardless of scrolling behaviour.
3. All dashes in a line are the same length.

**Minimal repro:**

Core 3.1 and Framework 4.8 demo projects here: https://github.com/ben-reilly/wpf-dash-render-test