Thumbnail checks and fixes

[EricClaeys]

• Check for 0-length input images and thumbnails and display appropriate message.
• Handle very narrow input images, for example, 1-pixel wide.

[EricClaeys]

Thanks @thomasjacquin. Several people with the Allsky Website on remote servers have been having problems with the servers not being able to create thumbnails, especially the video ones, since many (most?) hosted servers disable the exec() PHP call which we use. I need to get off my butt and implement the ability for the thumbnails to be generated on the Pi and uploaded to the remote server, which will solve the issue.

If you get a chance, try installing the home-page-customizations branch, which combines the two configuration files into one (which can be edited in the WebUI if the Website's on the Pi). It also allows users to add icons to the list on the left, and on the pop out on the right, by simply changing the .json configuration file. Users can also add a background image, add a link to their own web site, and show or hide the "Make your own" link at the bottom right.

And if you have even more time, take a look at the overlay editor Alex Greenfield has been working on. It's a drag and drop method of placing text and pictures on the images, and will be done in post processing, so we can remove all the overlay code from capture*.cpp.