FIX: Make widget blitting compatible with swapped canvas

[timhoffm]

First step towards improving blitting - towards #30503 and fixing #30575.

This PR proposes a minimal low-level API to store background information on the canvas.
It also adds higher-level methods save_blit_background() and load_blit_background() to AxesWidgets. Child classes can execrise managed background storage though this API.

I specifically would like to have feedback on the solution idea and API design. Note: This is not intended to be a solution for everything right away, but I would like to learn more on blitting through the case of widgets.

Open topics

• Check whether widgets act reasonably if the background is pulled under them. - As implemented now, they won't crash, but we must ensure that they can recover the background. It may be (to be checked) that they just loose the background.
• We still need to refactor useblit from statically including canvas blitting capability to dynamic handling.

Edit: Tested the following examples all with blitting enabled, and they work as expected (after closing the figure, plt.figure(fig); plt.show() recrates the figure and the widgets continue to work.

[timhoffm]

Recent update/commit: Support blitting with changing canvas in some widgets

Approach depends a bit on the widget, but generally:

• self._useblit only stores the flag passed to __init__
• canvas.supports_blit is checked dynamically when needed
• we sometimes have the property sef.useblit, which is the effective
  value of flag and capability
• if animated artists are used, that state is set during
  __init__ based on the flag. This relies on the fact that
  the flag cannot be changed later, and that the value of
  "animated" does not play a role for drawing canvases without blit support.

Supported:

• Button
• _SelectorWidget
• SpanSelector
• ToolLineHandles
• ToolHandles
• RectangleSelector
• LassoSelector
• PolygonSelector

Not supported yet:

• CheckButtons
• RadioButtons
• Cursor
• MultiCursor
• Lasso

This should already fix #30575 as Selector widgets are covered.

Note: It's probably best to review the commits individually.

[tacaswell]

Drat, I did not re-look at this PR before I opened #30605

[timhoffm]

Rebased.

[timhoffm]

@tacaswell How do we want to continue here? I consider this PR complete for the handled widgets, and suggest that we review and merge it. The remaining widgets should be handled in a follow-up PR to keep this PR more manageable.

Here's a summary of this PR approach:

• First commit: Create an infrastructure that safely handles background images by storing them on the canvas and defining an API in a way that there is no guarantee a background image is available. Consumers can try to obtain a background but must gracefully handle the case that no background is available.
  This is the basis to be able to reasonably handle blit-backgrounds while swapping canvas. Apply this to widgets that store backgrounds.
• Second commit: Fix dynamic changes in useblit on the individual Widgets case-by-case. Since logic slightly differs between widgets, we have to think them through individually. The list of currently fixed widgets is in FIX: Make widget blitting compatible with swapped canvas #30591 (comment).

[tacaswell]

Do we want to pass the bounding box in instead?

[timhoffm]

For symmetry reasons and to keep the changes within the PR minimal, I would leave the complete handling to the individual widgets for now.

They do

self._save_blit_background(self.canvas.copy_from_bbox(self.ax.bbox))

and

background = self._load_blit_background()
if background is not None:
    self.canvas.restore_region(background)

It may be reasonable to refactor in a follow-up and move that logic to the base class.