Revert "Revert "Revert "Revert "Don't build GL on Metal, Vulkan, Dawn, Direct3D bots""""

Updated to use sentinel GL context even when GL backend is not built.

This reverts commit 1171d314efc7e131b039c94126d26575cfb9e70c.

Change-Id: Ia94bbe4865ddd4e898446c13886877c539f0eb0b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/277976
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/tools/viewer/Viewer.cpp b/tools/viewer/Viewer.cpp
index b1c3819..665da4a 100644
--- a/tools/viewer/Viewer.cpp
+++ b/tools/viewer/Viewer.cpp
@@ -131,6 +131,10 @@
 
 static DEFINE_bool(offscreen, false, "Force rendering to an offscreen surface.");
 
+#ifndef SK_GL
+static_assert(false, "viewer requires GL backend for raster.")
+#endif
+
 const char* kBackendTypeStrings[sk_app::Window::kBackendTypeCount] = {
     "OpenGL",
 #if SK_ANGLE && defined(SK_BUILD_FOR_WIN)