hidpi: Really use BMP_SCALE_FAST when scaling the images.

It is not a problem of performance, but of the look - the images get too
blurry in the case of icons; and the blurry look is worse than than the
artifacts of the fast scaling.

Revert "hidpi: Use the default scaling algorithm."

This reverts commit e07097cce36f1220f5574a80dc22eeabb3005261.

Change-Id: I8af2827758e02ec3c8b7dade1559c45bd9f0ef35