Asset updates

[apeltsi]

Loitsu should be capable of updating assets. This would make hot-reloading possible.

Sprites may actually be able to just create a new texture on the GPU, then increment some version number so that all drawables holding a reference to the old version can update automatically.

Note that a texture updating can also mean that the UVs (see #27) could change. This cannot be done without the drawable doing some work, which could be done when it detects a texture change and updates its own texture.

[apeltsi]

This is required for the direct asset management model to work

[apeltsi]

In some cases the asset may be None at first, until it loads (direct asset-management model). So the type should be an Arc\<Mutex\<Option\<Asset>>> or something along those lines im boutta fall asleep