Improve the OpenGL context request system

src/lib.rs

@@ -114,6 +114,30 @@ pub enum Api {
     WebGl,
 }
 
+/// Describes the OpenGL API and version that are being requested when a context is created.
+#[derive(Debug, Copy, Clone)]
+pub enum GlRequest {
+    /// Request the latest version of the "best" API of this platform.
+    ///
+    /// On desktop, will try OpenGL.
+    Latest,
+
+    /// Request a specific version of a specific API.
+    ///
+    /// Example: `GlRequest::Specific(Api::OpenGl, (3, 3))`.
+    Specific(Api, (u8, u8)),
+
+    /// If OpenGL is available, create an OpenGL context with the specified `opengl_version`.
+    /// Else if OpenGL ES or WebGL is available, create a context with the
+    /// specified `opengles_version`.
+    GlThenGles {
+        /// The version to use for OpenGL.
+        opengl_version: (u8, u8),
+        /// The version to use for OpenGL ES.
+        opengles_version: (u8, u8),
+    },
+}
+
 #[derive(Debug, Copy)]
 pub enum MouseCursor {
     /// The platform-dependent default cursor.
@@ -194,7 +218,7 @@ struct BuilderAttribs<'a> {
     dimensions: Option<(u32, u32)>,
     title: String,
     monitor: Option<winimpl::MonitorID>,
-    gl_version: Option<(u32, u32)>,
+    gl_version: GlRequest,
     gl_debug: bool,
     vsync: bool,
     visible: bool,
@@ -215,7 +239,7 @@ impl BuilderAttribs<'static> {
             dimensions: None,
             title: "glutin window".to_string(),
             monitor: None,
-            gl_version: None,
+            gl_version: GlRequest::Latest,
             gl_debug: cfg!(ndebug),
             vsync: false,
             visible: true,