gl-renderer: add EGL surfaceless platform support

This allows passing EGL_PLATFORM_SURFACELESS_MESA to
gl_renderer_display_create(). It is not useful on its own, because the
surfaceless platform has no window surfaces.

This feature will be used by the headless backend.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

shared/weston-egl-ext.h

@@ -176,6 +176,10 @@ typedef EGLSurface (EGLAPIENTRYP PFNEGLCREATEPLATFORMPIXMAPSURFACEEXTPROC) (EGLD
 #define EGL_PLATFORM_X11_KHR 0x31D5
 #endif
 
+#ifndef EGL_PLATFORM_SURFACELESS_MESA
+#define EGL_PLATFORM_SURFACELESS_MESA     0x31DD
+#endif
+
 #ifndef EGL_KHR_cl_event2
 #define EGL_KHR_cl_event2 1
 typedef void *EGLSyncKHR;
@@ -210,6 +214,7 @@ typedef EGLint (EGLAPIENTRYP PFNEGLDUPNATIVEFENCEFDANDROIDPROC) (EGLDisplay dpy,
 #define EGL_PLATFORM_GBM_KHR     0x31D7
 #define EGL_PLATFORM_WAYLAND_KHR 0x31D8
 #define EGL_PLATFORM_X11_KHR     0x31D5
+#define EGL_PLATFORM_SURFACELESS_MESA     0x31DD
 
 #endif /* ENABLE_EGL */