Support high-resolution cursor textures

In original engine cursor uses a 3D mesh, so cursor texture is scaled to that mesh size. OpenMW uses a 2D SDL surface instead, and size of this surface matches a cursor texture resolution. As a result, any cursor replacer which does not use textures with 32x32 pixels resolution does not work properly.

Since we use SDL's software renderer to prepare a cursor surface, we can try just to change target surface size to match the expected cursor size, not an original texture size. Optionally we can make cursor to respect UI scale size, not sure about it.