[core] add raster-dem source type and hillshade layer type (#10642)

src/mbgl/programs/hillshade_program.cpp

@@ -0,0 +1,7 @@
+#include <mbgl/programs/hillshade_program.hpp>
+
+namespace mbgl {
+
+static_assert(sizeof(HillshadeLayoutVertex) == 8, "expected HillshadeLayoutVertex size");
+
+} // namespace mbgl