QJpegHandler is creating and destroying the jpeg (de)compressor multiple times

Currently QtWebKit's ImageDecoderQt.cpp is asking for the ImageSize, the ImageFormat and later decoding asking to decode the image itself. The QJpegHandler will call read_jpeg_format, read_jpeg_size and read_jpeg_image which will create and destroy the JPEG decompressor three times for every JPEG image.

The boilerplate code for read_jpeg_format and read_jpeg_size is the same and the extra init of the decompressor could be saved. On top of that even the read_jpeg_image could use the same decompressor. One thing to get right though is that one can set "scaling" on the image, so the jpeg_calc_output_dimensions method needs to be called from the read_jpeg_image with updated information. From a quick look at the libjpeg code this seems to be allowed and possible.

A quick hack showed a promising improvement on the "image_cycling" test case.