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.h

@@ -45,6 +45,7 @@ extern "C" {
 
 enum _egl_platform_type {
    _EGL_PLATFORM_X11,
+   _EGL_PLATFORM_XCB,
    _EGL_PLATFORM_WAYLAND,
    _EGL_PLATFORM_DRM,
    _EGL_PLATFORM_ANDROID,
@@ -294,6 +295,12 @@ _EGLDisplay*
 _eglGetX11Display(Display *native_display, const EGLAttrib *attrib_list);
 #endif
 
+#ifdef HAVE_XCB_PLATFORM
+typedef struct xcb_connection_t xcb_connection_t;
+_EGLDisplay*
+_eglGetXcbDisplay(xcb_connection_t *native_display, const EGLAttrib *attrib_list);
+#endif
+
 #ifdef HAVE_DRM_PLATFORM
 struct gbm_device;