fakevideosink: Allow allocation meta flags fine-tuning
In some scenarios the fakevideosink shouldn't advertize the overlay-composition meta for instance, so that overlay elements perform subtitles blending themselves.
Only advertizing the meta in the allocation query is insufficient, the metas are
not handled anyway. Hence fallback to subtitles overlay blending performed in
textoverlay.
The use-case for this is SSIM-based subtitles rendering checks in
gst-integration-test-suites.
Edited by Philippe Normand