evas - unload/scalecache self-feeding loop unload/reload fix

evas - unload/scalecache self-feeding loop unload/reload fix

i was runing perf top and noticed that evas_image_load_file_data_eet(0
was being called. in fact - it was #1 on the list of functions being
called. why? it didn't make sense. i found out. just a blinking cursor
in terminology was causing the background to be unloaded and
re-loaded. the new "actually unload" changes for 1.15 made this happen
and thus we kept sucking in new data all the time even if the
scalecache already had the data - and that was the problem.

so now calcecache prepare tells you if you don't have cached data and
if you likely then have to ensure the data is loaded. this cuts down
quite a bit of work.

while i'm at it... we definitely need to clean house on the internals
of evas. a decade+ of features, mess, optimizations needs to be fixed.
i mean really house-cleaned. rewritten clenl;y re-using existing code
where appropriate.