New functions bootLogoCompressed(), bootLogoSpritesSelfMasked() and
bootLogoSpritesOverwrite() can be used in place of bootLogo() to
reduce code size in cases where their drawing functions are shared
with the same functions used by the sketch.
New function bootLogoShell() added to provide common code for the
above functions.
Also, the Sprites class functions, and functions used for drawing the
logos, were made static.