Calculate window input event transform only on window change

[Sauermann]

Currently during every event processing, Viewport::_get_input_pre_xform() calculates the Transform2D, that is based on a few attributes of Windows like CONTENT_SCALE_ASPECT, CONTENT_SCALE_MODE and others.
However this Transform2D only changes, when the window changes.

With this patch, the Transform2D gets only recalculated after a window change.
This change also makes several transform-calculations in Viewport simpler.

My MRP for testing these changes consists of multiple subwindows, that can be configured with all available options:
WindowsTestbed.zip (updated 2022-05-07 since Input-API changed, updated 2022-09-18 since Transform2D-API changed)

Update 2022-10-01: split nonessential parts into other PRs

[Sauermann]

I noticed, that it is a bad idea to merge "Stretch Transform" into "Window transform", because one could imagine, that "Stretch Transform" could be changed without knowledge of Window, which would lead to unexpected behavior. My latest update solves this problem.

Extended MRP: WindowsTestbed.zip (no longer works because of Input-API changes)

[Sauermann]

Originally, this enhancement PR contained a bugfix. I have split this bugfix into its own PR: #59709

[akien-mga]

This has been updated / simplified, would be good to review further.

I'm not 100% clear on what are the actual benefits of this PR though. Is this a performance optimization, or does it solve issues?

[Sauermann]

Since I don't intend to change any functionality with this PR, it does not solve any issues.
My reasons for this PR are:

1. Structure: This transform originates in the Window. Viewport should be able to function, without knowing anything about it. Even SubViewports have to go through _get_input_pre_xform, even though it is always the identity transform for them.
2. Clarity: _get_input_pre_xform is not very descriptive and it took me a long time to understand, where it comes from and why it is necessary. I would like to make the functionality more accessible by moving the calculation to Window::_update_viewport_size, where the transform originates in.
3. Code simplification:

In a followup I want to remove Viewport::to_screen_rect. The only other place, where it is used, can be replaced by a check, if this is a Window or a SubViewport. It seems like to_screen_rect was only passed to Viewport in order to calculate the transform there.
In a followup after #66692, I want to replace Viewport::_get_input_pre_xform by a lookup in the Window class.

4. Performance: but I am not sure if this is for events necessary

[groud]

Approved in PR review meeting.

[akien-mga]

Thanks!