gdk: use GLES when on win32/ANGLE

When using GDK_DEBUG=gl-egl, we end up using GL, but that is not well supported:

Creating EGL context version 3.0 (debug:no, forward:no, legacy:yes, es:no)
Created EGL context[0000000000000004]
OpenGL version: 0.0 (legacy)
* GLSL version: (NULL)
* Max texture size: -1059701680
* Extensions checked:
 - GL_KHR_debug: no
 - GL_EXT_unpack_subimage: yes
 - OES_vertex_half_float: no

** (gtk4-demo.exe:14324): WARNING **: 19:16:41.468: Compile failure in
vertex shader:
ERROR: 0:7: 'gl_Position' : undeclared identifier
---8<---

Use GLES when EGL implementation is ANGLE.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>