Add opt-out environment variable for @2x images

There are cases where @2x images are in use but
where you don't want the associated behavior:
- You are implementing an image editor
- You are inheriting a resource set which has @2x
  images, but don't want that behavior with Qt.

Add support for disabling @2x behavior with QT_HIGHDPI_DISABLE_2X_IMAGE_LOADING.

Change-Id: I206e050b6241f8935b4404c7a28b024a9e00d46c
Task-id: QTBUG-38485
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>