Improvement of the ImageLayer class

[lesquoyb]

Is your request related to a problem? Please describe.
Following the PR #263, here are some pointers for improving the ImageLayer class:

1. This class uses a cached ImageProvider, but as the layer is only refreshed when the refresh facet is true, and that is checked in different parts of the code, I wonder what's the point of this object ? Maybe for files ? If none then it should be removed
2. The BufferedImage to be drawn is actually generated in the buildImage method, but it is not returned, instead the ImageProvider that did generate it is returned and is then drawn with the drawAsset method. I think that this method is generating another bufferedimage but I'm not sure and it may depend on many factors (nature of the provider, facets of the layer/display etc.). I did a test it with a matrix that changed every cycle and compared the average cycle time with current method and with one were we return the buffered image and draw it with the method drawImage instead: I noticed a slight improvement (around 10%) in the average cycle execution time but it may as well be normal variability in simulations and I didn't dig much more.