gluBuild2DMipmapsは任意のサイズの元画像を2^n乗のサイズに拡大縮小する

http://www.opengl.org/sdk/docs/man/xhtml/gluBuild2DMipmaps.xml
＞Initially, the width and height of data are checked to see if they are a power of 2.
＞If not, a copy of data (not data), is scaled up or down to the nearest power of 2.