Compressing normals in meshes.

Normals are now saved as three denormalized signed bytes + one byte
padding instead of three floats, saving 8 bytes per vertex. Total
resource file size is stripped down by almost 1/4, from 626 kB to 475
kB. The size could be reduced further by using vec3 instead of vec4 in
vertex shader.
Using Color3::lerp() instead of own code.

LERP in Magnum will be implemented using SIMD someday, so why not be
prepared for it.
Fixed swapped colors on animation stop.

animationStopped() wasn't called due to an bug, thus this issue was not
noticed until now.