FS-11117: font_scale float values, add min/max_font_size when setting video banner

FS-11117: font_scale float values, add min/max_font_size when setting video banner

Previously, the font_scale parameter available for the video_mute_banner/video_banner_text
conference channel variable would only accept integer values. It now can accept float
values for more fine grained control, eg:

  <action
    application="set"
    data="video_banner_text={font_scale=.75}Banner Text"
  />

Add params to set min_font_size and max_font_size for video banners.

The hard-coded defaults of 5/24 for min/max font size don't always look good
depending on the layout use case. This adds min_font_size and max_font_size
params for more fine-grained control, eg:

  <action
    application="set"
    data="video_banner_text={min_font_size=8,max_font_size=14}Banner Text"
  />