feat(assets): add a way to get current relative frame

This function return the current frame relative to the first frame of 
the current animation

CHANGELOG.md

@@ -13,6 +13,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - **actor2D:** Add multiple hitbox support.
 
+- **assets:** Add a way to get current relative frame
+
+
 ### Changed
 
 - **world2D:** Use a list for bodies (hitboxes, etc) and one other for actors

gamecore/modules/assets/animator.lua

@@ -102,6 +102,10 @@ function Animator:getFrame()
   return self.frame
 end
 
+function Animator:getRelativeFrame()
+  return self.frame - (self.animationData.startAt) + 1
+end
+
 function Animator:animationExist(name)
   return (self.sprite.data.animations[self.currentAnimation] ~= nil)
 end

gamecore/modules/assets/sprites.lua

@@ -74,6 +74,10 @@ function Sprite:getDimensions()
   return self.tileset:getDimensions()
 end
 
+function Sprite:getRelativeFrame()
+  return self.animator:getRelativeFrame()
+end
+
 -- DRAW FUNCTIONS
 -- Draw sprites using these functions