Improvements for the image layer

[lesquoyb]

fixes #262 but also some clues for further improvements.
@AlexisDrogoul could you double check this one as there are some points that I think could be improved in this class but I'm not sure yet:

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 ?
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.

[codescene-delta-analysis]

✅ Code Health Quality Gates: OK

Change in average Code Health of affected files: -0.00 (9.60 -> 9.59)

View detailed results in CodeScene

[codescene-delta-analysis]

✅ Code Health Quality Gates: OK

Change in average Code Health of affected files: -0.00 (9.57 -> 9.57)

View detailed results in CodeScene