EGL: Add support for EGL_ANGLE_platform_angle

This adds basic support for selecting the platform type (rendering
backend) when running on ANGLE.

Related to #1380.

src/init.c

@@ -53,6 +53,7 @@ static GLFWerrorfun _glfwErrorCallback;
 static _GLFWinitconfig _glfwInitHints =
 {
     GLFW_TRUE,      // hat buttons
+    GLFW_ANGLE_PLATFORM_TYPE_NONE, // ANGLE backend
     {
         GLFW_TRUE,  // macOS menu bar
         GLFW_TRUE   // macOS bundle chdir
@@ -287,6 +288,9 @@ GLFWAPI void glfwInitHint(int hint, int value)
         case GLFW_JOYSTICK_HAT_BUTTONS:
             _glfwInitHints.hatButtons = value;
             return;
+        case GLFW_ANGLE_PLATFORM_TYPE:
+            _glfwInitHints.angleType = value;
+            return;
         case GLFW_COCOA_CHDIR_RESOURCES:
             _glfwInitHints.ns.chdir = value;
             return;