widgets: Fix some issues

[mattst88]

• widgets: correct OH_Partner_frame.png dimensions, metadata, and test fixtures
• widgets: correct Tiny widget height to match profile_tiny.png (15 → 16)
• widgets: add PNG output format support

  `?format=gif` actually emits a PNG but with a Content-Type of image/gif.
  
  So add `?format=png` which will emit the same underlying image data as
  before but with the correct Content-Type of image/png.
  

• widgets: flatten transparent background to white

  OH_Partner_frame.png has an alpha channel — the content area was
  transparent. The kudos badge composited on top has a white background,
  making it visually inconsistent. Flatten to white so the entire widget
  has a uniform opaque background.
  

[mattst88]

cc: @Priya5

[mattst88]

Can someone review this? It's pretty trivial.

cc: @Niharika1117