use content-type hint for better encoding selection and tuning

[Antoine Martin]

See #1950: we should try to use the content-type hint more when selecting an encoding.

ie:

• lower lossless threshold for "text", increase it for "video".
• prefer "webp" when we have a content type hint since this is only encoder to actually take this hint into account during compression

etc

[Antoine Martin]

See also ticket:1951#comment:1, as of r20366 "video" content-type will trigger more video-scaling.

[Antoine Martin]

Updates:

• r20375: better tuning for 'text' and 'video' content-type
• r20374: allows us to set the default content-type: XPRA_DEFAULT_CONTENT_TYPE=text xpra start ..
• r20376 + r20383: avoid using rgb for large areas (512 KPixels or more) unless speed is at 100%
• r20392: use 'grain' x264 tune setting for text content

[Antoine Martin]

See also #2022, #2023

[Antoine Martin]

r20911: try harder to honour video content-type

[Antoine Martin]

Many more updates in r25902.