Depending on the underlying EGL library, it may be desirable to
conditionally set some specific EGL attributes depending on available
extensions and other application state.
SDL's EGL usage makes this a little bit complicated because:
- there are multiple functions used to set up a working EGL context
- some of these functions take different types of EGL attributes
(EGLAttrib vs EGLint)
- the EGL extension list before creating an EGLDisplay differs from the
extension list after (i.e. display vs client extensions)
- all of the above happens in a single SDL_CreateWindow call
This leaves no place for the application to discover what EGL extensions
are available and provide custom attribute lists.
Until now, if a developer wants to add a custom EGL attribute for
eglGetPlatformDisplay, eglCreateWindowSurface or eglCreateContext, they
needed to patch SDL itself. This is very undesirable, since such
developers would have to disable the SDL dynapi in order to maintain
compatibility with their needs.
This patch implements some callbacks which developers can use to
dynamically generate custom EGL attributes for SDL to use during
SDL_CreateWindow.
These functions allow applications to call EGL functions against the SDL
EGL context. For example, applications can use an EGL API loader via
SDL_EGL_GetCurrentDisplay and SDL_EGL_GetProcAddress, and can call
functions such as eglQuerySurface against the internal EGLSurface and
EGLDisplay.
This implements a new SDL_GL_EGL_PLATFORM attribute to set the
"platform" argument for SDL_EGL_LoadLibrary on Windows, macOS, and
Linux. I've limited it to those three operating systems because that's
what I've been able to test against.
- This allows looking up the display index for an arbitrary location rather than requiring an active window to do so.
- This change also reimplements the fallback display lookup that found the display with center closest to the window's center to instead find the display rect edge
closest to the window center (this was done in the almost identical display lookup used in SDL_windowsmodes.c, which now uses `SDL_GetPointDisplayIndex`). In
practice this should almost never be hit as it requires the window's center to not be enclosed by any display rect.
Extend the SDL_WINDOW_ALLOW_HIGHDPI flag documentation to stress the importance of querying the window drawable size after every window event to avoid rendering issues in a mixed-DPI environment.
This API and implementation comes from the Unreal Engine branch of SDL, which
originally called this "SDL_ConfineCursor".
Some minor cleanup and changes for consistency with the rest of SDL_video, but
there are two major changes:
1. The coordinate system has been changed so that `rect` is _window_ relative
and not _screen_ relative, making it easier to implement without having
global access to the display.
2. The UE version unset all rects when passing `NULL` as a parameter for
`window`, this has been removed as it was an unused feature anyhow.
Currently this is only implemented for X, but can be supported on Wayland and
Windows at minimum too.
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