Fix heightmap shadow rt and draw the light pos

src/app_shaders.cpp

@@ -548,7 +548,7 @@ void App::initShaders()
         "in mediump vec2 uv;\n"
         "out mediump vec4 frag;\n"
         "void main() {\n"
-        "   mediump float d = max(0.0, dot(normalize(n), light_dir));\n"
+        "   mediump float d = max(0.0, dot(normalize(n), normalize(light_dir)));\n"
         "   mediump vec4 c = texture(tex, uv);\n"
         "   frag = vec4(c.rgb * d, 1.0);\n"
         "}\n";