Add mouse tracking, add null-texture gen function

Refactor the creation of the null-texture into a function. Used for
blitting rectangles that don't have any texture associated with it.

src/Renderer.c

@@ -1,4 +1,6 @@
 #include "Dengine/Renderer.h"
+#include "Dengine/AssetManager.h"
+#include "Dengine/Assets.h"
 #include "Dengine/Debug.h"
 #include "Dengine/Entity.h"
 #include "Dengine/OpenGL.h"
@@ -138,6 +140,13 @@ INTERNAL void renderObject(Renderer *renderer, v2 pos, v2 size, v2 pivotPoint,
 	GL_CHECK_ERROR();
 }
 
+RenderTex renderer_createNullRenderTex(AssetManager *assetManager)
+{
+	Texture *emptyTex = asset_getTexture(assetManager, texlist_empty);
+	RenderTex result  = {emptyTex, V4(0, 1, 1, 0)};
+	return result;
+}
+
 void renderer_rect(Renderer *const renderer, v4 cameraBounds, v2 pos, v2 size,
                    v2 pivotPoint, f32 rotate, RenderTex renderTex, v4 color)
 {