fix lerp for vec3, update tests for broken lerp case

modules/utils.lua

@@ -74,7 +74,7 @@ end
 -- @param progress (0-1)
 -- @return number
 function utils.lerp(low, high, progress)
-	return (1 - progress) * low + progress * high
+	return low * (1 - progress) + high * progress
 end
 
 --- Exponential decay

spec/utils_spec.lua

@@ -16,6 +16,11 @@ describe("utils:", function()
 		local b = vec3(1, 1, 1)
 		local c = vec3(0.5, 0.5, 0.5)
 		assert.is.equal(utils.lerp(a, b, 0.5), c)
+
+		a = vec3(5, 5, 5)
+		b = vec3(0, 0, 0)
+		c = vec3(2.5, 2.5, 2.5)
+		assert.is.equal(utils.lerp(a, b, 0.5), c)
 	end)
 
 	it("decays exponentially", function()