Multimedia: describe single-image DMABuf textures by pixel format

Single-image DMABuf imports expose one texture for a multi-plane frame.
The handles used to carry a texture count, with -1 meaning one per
plane, plus a separate external OES flag. Replace both with the pixel
format the handles' textures represent: single-image imports report
Format_RGBA8888, or Format_SamplerExternalOES when external OES is
forced. The texture count and the ExternalOES flag now follow from
that format.

Reject single-image imports for formats without a single-image DRM
fourcc instead of silently using the first plane's format, and take
the handle textures as a vector so their count is its size.

Pick-to: 6.12
Change-Id: I9e1d8b59868336e964ca673fb5359ff8dc4817ca
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>