This commit addresses warnings in the OpenXR rendering pipeline that
occurred immediately after initialization. In such cases, when the
`XrFrameState.shouldRender` property, used to set the `ShouldRenderCurrentFrame`
flag, evaluates to False, we eliminate explicit use of the `vtkWarningMacro`.
This prevents warnings like the following from being reported after
a standard pipeline initialization:

```
Warning: In vtkOpenXRRenderWindow.cxx, line 189
vtkOpenXRRenderWindow (0000028A641868D0): Not rendered
```

This resolves issue #19112 (OpenXR Renderer: always warn "Not rendered"
at startup). Additionally, the associated docstring for the
`ShouldRenderCurrentFrame` instance variable has been updated for clarity.

Furthermore, the flag "ShouldRenderCurrentFrame" is proactively reset to avoid
any attempted rendering if the function `WaitAndBeginFrame()`, used in
`vtkOpenXRRenderWindow::Render`, exits prematurely.

For more details on `XrFrameState` and `XrFrameBeginInfo`, refer to the
OpenXR specification:
- https://registry.khronos.org/OpenXR/specs/1.0/man/html/XrFrameState.html
- https://registry.khronos.org/OpenXR/specs/1.0/man/html/XrFrameBeginInfo.html