Fix drill summary problem with multiple drill files
Brought to you by:
spetm,
thepurlieu
When there is more than one drill file loaded (eg, PTH and NPTH) the drill summary dialog is wrong - it shows sizes from one file with counts from the other. This fixes that, now all the files are effectively merged in the summary. Same size drills from different files have their counts merged; they could be shown separately but that could be confusing.
drill_stats_add_to_drill_counter() is libgerbv function, its argument number can't be changed that easily.