egl: support ANGLE on Windows

Angle provides libEGL.dll, which glutin loads. The only change necessary
is to get the platform display via EGL_PLATFORM_ANGLE_ANGLE and treating it
as a legacy display.

Fixes #1508.

CHANGELOG.md

@@ -1,5 +1,7 @@
 # Unreleased
 
+- Added support for EGL on Windows using Angle. This assumes libEGL.dll/libGLESv2.dll present.
+
 # Version 0.30.9
 
 - Fixed lock on SwapBuffers with some GLX drivers.

glutin/src/api/egl/display.rs

@@ -241,6 +241,7 @@ impl Display {
         let extensions = NO_DISPLAY_EXTENSIONS.get().unwrap();
 
         let mut attrs = Vec::<EGLint>::new();
+        let mut legacy = false;
         let (platform, mut display) = match display {
             #[cfg(wayland_platform)]
             RawDisplayHandle::Wayland(handle)
@@ -266,6 +267,11 @@ impl Display {
             RawDisplayHandle::Gbm(handle) if extensions.contains("EGL_MESA_platform_gbm") => {
                 (egl::PLATFORM_GBM_MESA, handle.gbm_device)
             },
+            RawDisplayHandle::Windows(..) if extensions.contains("EGL_ANGLE_platform_angle") => {
+                // Only CreateWindowSurface appears to work with Angle.
+                legacy = true;
+                (egl::PLATFORM_ANGLE_ANGLE, egl::DEFAULT_DISPLAY as *mut _)
+            },
             _ => {
                 return Err(
                     ErrorKind::NotSupported("provided display handle is not supported").into()
@@ -284,7 +290,17 @@ impl Display {
         let display =
             unsafe { egl.GetPlatformDisplayEXT(platform, display as *mut _, attrs.as_ptr()) };
 
-        Self::check_display_error(display).map(EglDisplay::Ext)
+        Self::check_display_error(display).map(|display| {
+            if legacy {
+                // NOTE: For angle we use the Legacy code path, as that uses CreateWindowSurface
+                // instead of CreatePlatformWindowSurface*. The latter somehow
+                // doesn't work, only the former does. But Angle's own example also use the
+                // former: https://github.com/google/angle/blob/main/util/EGLWindow.cpp#L424
+                EglDisplay::Legacy(display)
+            } else {
+                EglDisplay::Ext(display)
+            }
+        })
     }
 
     fn get_display(egl: &Egl, display: RawDisplayHandle) -> Result<EglDisplay> {

glutin/src/display.rs

@@ -417,6 +417,11 @@ pub enum DisplayApiPreference {
     ///
     /// But despite this issues it should be preferred on at least Linux over
     /// GLX, given that GLX is phasing away.
+    ///
+    /// # Platform-specific
+    ///
+    /// **Windows:** ANGLE can be used if `libEGL.dll` and `libGLESv2.dll` are
+    ///              in the library search path.
     #[cfg(egl_backend)]
     Egl,
 

glutin_egl_sys/src/lib.rs

@@ -32,6 +32,16 @@ pub mod egl {
     pub const PLATFORM_XCB_SCREEN_EXT: super::EGLenum = 0x31DE;
     // EGL_EXT_device_query_name
     pub const RENDERER_EXT: super::EGLenum = 0x335F;
+    // EGL_ANGLE_platform_angle - https://chromium.googlesource.com/angle/angle/+/HEAD/extensions/EGL_ANGLE_platform_angle.txt
+    pub const PLATFORM_ANGLE_ANGLE: super::EGLenum = 0x3202;
+    pub const PLATFORM_ANGLE_TYPE_ANGLE: super::EGLenum = 0x3203;
+    pub const PLATFORM_ANGLE_MAX_VERSION_MAJOR_ANGLE: super::EGLenum = 0x3204;
+    pub const PLATFORM_ANGLE_MAX_VERSION_MINOR_ANGLE: super::EGLenum = 0x3205;
+    pub const PLATFORM_ANGLE_DEBUG_LAYERS_ENABLED: super::EGLenum = 0x3451;
+    pub const PLATFORM_ANGLE_NATIVE_PLATFORM_TYPE_ANGLE: super::EGLenum = 0x348F;
+    pub const PLATFORM_ANGLE_TYPE_DEFAULT_ANGLE: super::EGLenum = 0x3206;
+    pub const PLATFORM_ANGLE_DEVICE_TYPE_HARDWARE_ANGLE: super::EGLenum = 0x320A;
+    pub const PLATFORM_ANGLE_DEVICE_TYPE_NULL_ANGLE: super::EGLenum = 0x345E;
 }
 
 pub use self::egl::types::{EGLContext, EGLDisplay};