Multi-window support in Sdl2Application and GlfwApplication

[mosra]

Work-in-progress implementation of #124, option 3.

There's no backwards-incompatible change, except for Configuration::WindowFlag::Contextless and related flags, which are now application-global as supporting some windows with GL, some with Vulkan and some with nothing at all seems like a very rare use case that'd add a lot of implementation pain.

A new Sdl2ApplicationWindow class allows you to create another window for your application. Just creating the instance pops up a new window and all events that happen on it are transferred to events in that window.

Things left to do:

• Sdl2Window is probably a better name than Sdl2ApplicationWindow
• Per-window event handling
  • SDL_WINDOWEVENT_CLOSE for closing the window itself -- there's a conflict between leaving window lifetime management in users hands and "the close button just working", currently clicking the close button does nothing
    • maybe just have a closeEvent() which implicitly deletes the window but leaves the instance alive, which will make it no longer respond to events, and then the user is responsible for deleting the instance?
      • that also means all cases that now check for _windows[i] have to check for _windows[i]->_window too, the Application destructor also needs to verify not just that the SDL windows are deleted but that the Window wrapper instances are gone too so they don't access the Application after ... hmm but what if SDL would then reuse window IDs??
      • this could also be (ab)used for recreating the window later, i.e. the window-specific data being always there but the actual window only sometimes
  • Why there's no windowID for multi-gesture events? -- https://discourse.libsdl.org/t/how-to-get-the-origin-window-of-a-touch-event/19499/10, "impossible", move the event handler back to the app and document that
• Properly making GL context current for window that received the event
  • is the context switch expensive even if it's just the surface being changed?
  • do it only for drawEvent(), i.e. accessing the default framebuffer outside of it has unspecified behavior
    • and viewportEvent()? and tickEvent()? or not that one
• Figure out how to implicitly share some properties (like all windows being GL-enabled / Vk-enabled... by default if the main one was)
• Properly tracking framebuffer viewport state
• Properly handle events that don't belong to any window (e.g. mouse drag out) -- currently crashes or is ignored
• Figure out behavior with VSync (especially avoiding the refresh rate halving when another window gets opened)
  • would drawing all windows first and then calling the swapBuffers() only for the main one solve this?
  • is it still a problem with a single context?
  • https://stackoverflow.com/questions/29617370/multiple-opengl-contexts-multiple-windows-multithreading-and-vsync
• Ensure nothing breaks for GL-less builds
• Documentation
  • A new section in the Sdl2Application class docs
  • Documenting the ApplicationWindow typedef
  • Documenting shortcomings and bugs (one window has to be primary, the other windows not shown in taskbar, obviously no support on iOS/Emscripten)
  • Windows that don't belong to any window
  • Example -- [WIP] Example for Sdl2Application multi-window functionality. magnum-examples#25
• Test on Windows & OSX
• Test that single-window apps are not affected by any of these changes
• Ability to show/hide/raise the windows
  • And a WindowEvent to allow tracking this? Currently this all falls back to anyEvent().
• Ability to minimize/maximize them
• Ability to set and/query window positions
• Assert that additional windows are not created before the main window context is created
• All this for GLFW as well
• Is there something like multi-canvas for WebGL?
• Ensure assertions mention Sdl2ApplicationWindow instead of Sdl2Application where appropriate

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b1ba1f0) 82.25% compared to head (6394c85) 82.25%.

❗ Current head 6394c85 differs from pull request most recent head 17c74b2. Consider uploading reports for the commit 17c74b2 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #168      +/-   ##
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- Coverage   82.25%   82.25%   -0.01%     
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  Files         566      566              
  Lines       43967    43966       -1     
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- Hits        36167    36166       -1     
  Misses       7800     7800              

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