Masking fixes

Dear All,

I've just uploaded patch for:

BATIK-1170 - Workaround for incorrect ColorConvertOp alpha handling breaks
masking
https://issues.apache.org/jira/browse/BATIK-1170

I created this 'umbrela' issue as I found this is the root cause of various
symptoms reported in following issues:

BATIK-1080 - mask Element broken
BATIK-1105 - Incorrect mask rendering
BATIK-1131 - Mask cuts inappropriate region
BATIK-1156 - Batik 1.8 fails to rasterize SVG with mask/url
BATIK-1158 - <mask> using <filter> not rendered correctly
BATIK-1164 - Coordinates shift when filter is applied

Briefly, a specific workaround had to be applied for proper alpha processing
in old JDKs, however, since JDK8, which fixed the original problem, on the
contrary this workaround produced weird artefacts.

Please review my code. Any comments are welcome.

Thanks,

Jan

PS: Could admin somehow close listed issues as duplicate? (which is not even
correct as my issue has been created last).