Expose ImageBrush source region constructor.

[JimBobSquarePants]

Prerequisites

• [x] I have written a descriptive pull-request title
• [x] I have verified that there are no overlapping pull-requests open
• [x] I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules :cop:.
• [x] I have provided test coverage for my change (where applicable)

Description

See #305 The public constructor is required to selecting the source region providing the ability to negative offset and image.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (2316146) 80% compared to head (3ee51ef) 80%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #306 +/- ## =================================== Coverage 80% 80% =================================== Files 97 97 Lines 4946 4946 Branches 884 884 =================================== Hits 3997 3997 Misses 754 754 Partials 195 195 ``` | [Flag](https://app.codecov.io/gh/SixLabors/ImageSharp.Drawing/pull/306/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SixLabors) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/SixLabors/ImageSharp.Drawing/pull/306/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SixLabors) | `80% <100%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SixLabors#carryforward-flags-in-the-pull-request-comment) to find out more.

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