Dead assignment in WpfGfx handle_errors_and_transform_hresult

* .NET Core Version: N/A
* Windows version: N/A
* Does the bug reproduce also in WPF for .NET Framework 4.8?: No
* Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? No
* Security issues and bugs should be reported privately, learn more via our [responsible disclosure guidelines](https://github.com/dotnet/wpf/blob/master/README.md#reporting-security-issues-and-security-bugs).

**Problem description:**

The assignment to retval in handle_errors_and_transform_hresult, defined in shader_compiler_dx9.hpp and shader_compiler_xm.hpp, is unused. As a result, this function returns the error code passed to it in all cases, instead of changing some codes to WGXERR_SHADER_COMPILE_FAILED, as intended.

The correct solution is probably to return retval as intended, but I don't know if this will have any unintended consequences.

**Actual behavior:**

Compilation with mingw warns of an unused assignment to retval.

**Expected behavior:**

No compiler warnings.

**Minimal repro:**

I'm attempting to compile with mingw and a lot of changes were required to get this far. As such, I do not have a small way to test this.