v1.21.9

Changes between v1.21.8 and v1.21.9:
e34b3e0 log errors when writing to save file
42cda6d do not write to log file if cannot open
52063ac include errno
20fc64a avoid a warning when rendering zero length strings
23cd490 remove a debug print when exporting
dca728b add -no-border option to pack command
cb67cca add quads:clear() method
ad51f2f update copyright year
e328313 set android:resizeableActivity to false in AndroidManifest.xml
02b3304 keep local references to debug library in case it is redefined by user