backend: egl: fix creating eglpixmap

according to the specification of the EGL_KHR_image_pixmap extension,
if target is EGL_NATIVE_PIXMAP_KHR then ctx must be EGL_NO_CONTEXT,
otherwise, the EGL_BAD_PARAMETER error is generated. source:
https://registry.khronos.org/EGL/extensions/KHR/EGL_KHR_image_pixmap.txt

fixes #981

src/backend/gl/egl.c

@@ -277,7 +277,8 @@ egl_bind_pixmap(backend_t *base, xcb_pixmap_t pixmap, struct xvisual_info fmt, b
 
 	eglpixmap = cmalloc(struct egl_pixmap);
 	eglpixmap->pixmap = pixmap;
-	eglpixmap->image = eglCreateImageProc(gd->display, gd->ctx, EGL_NATIVE_PIXMAP_KHR,
+	eglpixmap->image =
+	    eglCreateImageProc(gd->display, EGL_NO_CONTEXT, EGL_NATIVE_PIXMAP_KHR,
 	                       (EGLClientBuffer)(uintptr_t)pixmap, NULL);
 	eglpixmap->owned = owned;