Add ability to save .rdc file thumbnail in formats other than JPEG

By default RenderDoc saves captured frame thumbnail image as JPEG which
is lossy compression and makes it difficult to use as a reference image
for testing. Added an option to save thumbnails in different formats.

In order not to change the binary serialization format of .rdc files, if
the thumbnail format is not JPEG, a default JPEG thumbnail is still
saved in the file header as before, while the actual thumbnail is saved
as an additional chunk in the binary sections list. Older versions of
RenderDoc will just ignore this section and still use the legacy JPEG
thumbnail.

Change-Id: Icbf4fbd629719b49868fb785a656f6c7c9946ef9