use global to pass compression progress

before this change threads could compete and the progress bar would jitter
report texture cache size, and use buttons to rebuild and clear cache
Merge branch 'master' of https://github.com/OpenCPN/OpenCPN
Correct occasional failure to wheel-zoom in smooth zoom mode.
Merge pull request #221 from seandepagnier/master

Improvements to texture cache user interface
Fix possible deref of a prefiously freed pointer
Fix possible resource leaks
Fix possible resource leaks for incorrect KAP charts
Fix possible resource leaks
Fix resource leak on failure to load ENC chart update
Remove unnecesary check confusing code analysis tools
Fix the comparison - 0 is a valid file descriptor and would be leaked if returned
Fix leak on failure to open an ENC chart
Fix string overflow and possible out of bounds access
Fix a string overflow while slightly improving performance
Merge pull request #222 from nohal/cov

Fixes for some problems discovered by code analysis
Implement Options setting for display size to accurately calculate onscreen scale.
Add Toggle FullScreen item to context menu if in FullScreen mode.
Correct scale calculations for rotated view.
Correct quilt mode toggle action persistence.