内存消耗计算方式 ：假如一张图是 1024 * 888
ETC1内存消耗 = 1024 * 888 * 4bpp / 8 = 454656byte = 444KB
PVRTCI_4BPP_RGBA内存消耗 = 1024 * 888 * 4bpp / 8 = 454656byte = 444KB
RGB888内存消耗 = 1024 * 888 * 24bpp / 8 = 2727936byte = 2664KB
RGBA8888内存消耗 = 1024 * 888 * 32bpp / 8 = 3637248byte = 3552KB
RGBA4444内存消耗 = 1024 * 888 * 16bpp / 8 = 1818624byte = 1776KB