plugins: Add more check for allowed raw caps.

He Junyan requested to merge He_Junyan/gstreamer-vaapi:fix_regression into master

The gst_vaapi_plugin_base_get_allowed_raw_caps is used for both sink pad and src pad, which cause some bugs. For sink pad, we need to verify vaPutImage() while for the src pad we need to verify vaGetImage(). For vaapidecoderXXX kind of plugins, the case is more complex. We need to verify whether the decoded result(in some surface, NV12 format most of the time) can be vaGetImage to some raw image format. Add more check to fix all these problems.

#123 (closed)

Signed-off-by: He Junyan

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