Health and Mana bar backgrounds

How to use:

1. Edit mods/healthbars.otmod and set autoload to true

2. Add g_game.enableFeature(GameHealthInfoBackground) to modules/game_features/features.lua

3. Add server side support by following this commit TODO

4. Add new backgrounds to mods/game_healthbars/healthbars.lua

Documentation:

g_healthBars.addHealthBackground(imagePath, imageOffsetX, imageOffsetY, barOffsetX, barOffsetY, barHeight)
g_healthBars.addManaBackground(imagePath, imageOffsetX, imageOffsetY, barOffsetX, barOffsetY, barHeight)

imagePath - path to image
imageOffsetX - move image to left/right
imageOffsetY - move image to top/bottom
barOffsetX - move health/mana bar to left/right
barOffsetY - move health/mana bar to top/bottom
barHeight - change health/mana bar height (default 4)