imgPNG.c: set screen_gamma to 2.2

It seems that gamma correction was never effective until the recent
change in imgPNG.c. But this change revealed that the former
screen_gamma value (1.0) was making images to dark on a normal PC
monitor.

Hardcoding to 2.2 is probably not correct on all platforms. But at
least PNG images look now the same like in 804.031.
fix clang compilation error

Missing return value in TclObj_free.
Merge branch 'master' into XXX-travis-ci