Add EGL xcb platform

This enables GL applications to be written without any involvement of
Xlib.

EGL X11 platform is actually already xcb-only underneath, so this commit
just add the necessary interface changes so eglDisplay can be created
from a xcb_connection_t.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6474>

src/egl/main/egldisplay.c

@@ -70,6 +70,7 @@ static const struct {
    const char *name;
 } egl_platforms[] = {
    { _EGL_PLATFORM_X11, "x11" },
+   { _EGL_PLATFORM_XCB, "xcb" },
    { _EGL_PLATFORM_WAYLAND, "wayland" },
    { _EGL_PLATFORM_DRM, "drm" },
    { _EGL_PLATFORM_ANDROID, "android" },
@@ -506,6 +507,27 @@ _eglGetX11Display(Display *native_display,
 }
 #endif /* HAVE_X11_PLATFORM */
 
+#ifdef HAVE_XCB_PLATFORM
+_EGLDisplay*
+_eglGetXcbDisplay(xcb_connection_t *native_display,
+                  const EGLAttrib *attrib_list)
+{
+   /* EGL_EXT_platform_xcb recognizes exactly one attribute,
+    * EGL_PLATFORM_XCB_SCREEN_EXT, which is optional.
+    */
+   if (attrib_list != NULL) {
+      for (int i = 0; attrib_list[i] != EGL_NONE; i += 2) {
+         if (attrib_list[i] != EGL_PLATFORM_XCB_SCREEN_EXT) {
+            _eglError(EGL_BAD_ATTRIBUTE, "eglGetPlatformDisplay");
+            return NULL;
+         }
+      }
+   }
+
+   return _eglFindDisplay(_EGL_PLATFORM_XCB, native_display, attrib_list);
+}
+#endif /* HAVE_XCB_PLATFORM */
+
 #ifdef HAVE_DRM_PLATFORM
 _EGLDisplay*
 _eglGetGbmDisplay(struct gbm_device *native_display,