Sprites are not animated in halley-test-entity

[jdecourval]

This is really hard to see with the default number of entities, but halley-test-entity example renders the whole spritesheet instead of an animation. Here is a screenshot with a single entity.

• Tested on current master
• archlinux
• GCC 9.1
• CMake 3.15.1
• boost 1.69.0
• freetype 2.10.1
• SDL2 2.0.10