Implement user-adjustable compression level

[avnishb]

Compression quality is currently fixed at 75 in Fusion's JPEGCompressor:

https://github.com/google/earthenterprise/blob/master/earth_enterprise/src/fusion/rasterfuse/ImageryPackgenTraverser.cpp#L40

This may result in noticeable artifacts, relative to the original source imagery, when serving 2D map tiles. So it might make sense to support user-specified, internal compression levels such that the Fusion administrator can set it based on tradeoff between image quality vs. storage size.

Note, however, there are other operations, such as reprojection when generating the pyramid files, that might also contribute to artifacts (depending on the source projection).