stub render backend for tests; hoist out raddbg code/entry points from build target file, to be called into by sanity tests

src/render/render_inc.c

@@ -4,7 +4,9 @@
 #include "render_core.c"
 
 #if LANG_CPP
-# if R_BACKEND == R_BACKEND_D3D11
+# if R_BACKEND == R_BACKEND_STUB
+#  include "stub/render_stub.cpp"
+# elif R_BACKEND == R_BACKEND_D3D11
 #  include "d3d11/render_d3d11.cpp"
 # else
 #  error Renderer backend not specified.