bootanimation: performance enhancements

This patch implements two simple performance enhancements, which
might benefit devices. They are completely opt-in by setting
the following variables.

  * TARGET_BOOTANIMATION_PRELOAD: will preload the bootanimation
    zip to memory; this is handy on devices where the storage
    might be slow. By preloading it, we get quick access to the
    zip file, and the animation doesn't stutter.

  * TARGET_BOOTANIMATION_TEXTURE_CACHE: allows maintainers to
    force texture cache on/off.

Also fixes the sign warnings related to int / size_t comparison

Change-Id: Ibfd04caf019bcac9ba9f80f8c3eadca397a998d1