Introduce two new environment variables which control the maximum amount of buffered data by the downloadbuffer element. This is useful in case it is necessary to control buffering large files, way larger than the default downloadbuffer limits. The following new variables are added:
    
- WPE_SHELL_MEDIA_DISK_CACHE_SIZE_BYTES ~ downloadbuffer max-size-bytes
- WPE_SHELL_MEDIA_DISK_CACHE_SIZE_NSEC ~ downloadbuffer max-size-time
    
Example HTML file, note esp. the 'preload="auto"' which is mandatory:
```
<video src="./file.mp4" autoplay muted loop preload="auto"></video>
```
    
Example invocation to cache up to 200 MiB per file and 500s per file:
```
$ WPE_SHELL_MEDIA_DISK_CACHE_SIZE_BYTES=268435456 \
  WPE_SHELL_MEDIA_DISK_CACHE_SIZE_NSEC=500000000000 \
  WEBKIT_GST_USE_PLAYBIN3=1 \
  cog http://example/index.html
```