[common_c] Hoist declarations in draw_battery.c
[common_c] Cache the path and destroy the drawing context.

Should fix a major memory leak I've known existed since I wrote it.
Honestly, I just didn't care when I intiially wrote the code, because I
didn't expect it to actually power anything.
[common_c] Remove unused extents stuff in generate_battery_text
Merge pull request #6 from NuckChorris/fix-memory-leaks

Fix memory leaks