gdk: prefer GLES when on win32/ANGLE

Merged Marc-André Lureau requested to merge malureau/gtk:angle-gles into main

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<---

Prefer GLES when EGL implementation is ANGLE.

Signed-off-by: Marc-André Lureau

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