bootanim: Don't cache textures if they're expected to use a lot of VRAM

bootanimation cached all the textures generated until the animation
was completed.
For Hi-res animations with lots of frames (like the new CM9 anim on
xhdpi devices), this implied using large amounts of video RAM, more
than most devices actually have available, causing the animation to
stall (and in some cases, gralloc to lock up); so if an animation is
expected to use a lot of RAM (rough estimation based on frame size
and number of frames), disable cache entirely and generate the textures
on demand

Change-Id: I157e5cdde2aab1d82d980d77cbedd9f127c83a1d

Conflicts:

	cmds/bootanimation/BootAnimation.cpp
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: Ie92d1b0aa90b43ba39c368a2a33657a60c3a64b5

Conflicts:

	cmds/bootanimation/BootAnimation.cpp
bootanimation: allow using RGB565 instead of ARGB8888

RGB565 uses considerably less memory, and together with texture
cache, it allows us to offer a really smooth bootanimation on
not so powerful devices.

Change-Id: I9da8fd7e6a587b5895519dd0983ec9b8f676771b