Add converted PDF output size to options

[lehik]

When importing PDFs, FreeShow seems to follow the original document sizing when converting to images. Sometimes this produces unnecessarily large files if the originals are hi-dpi documents. It would be convenient if you could define maximum output size in the options (for example 1920x1080) to make using the images a bit lighter and have image size identical to the output size to avoid the need to scale in real time.