Add a property to clear texture at disconnection

[norihiro]

Description

Add a property "Clear texture at disconnection".

When the property is enabled, calls obs_source_output_video with frame = NULL so that the texture will be deactivated.

How Has This Been Tested?

Checked the texture size became 0x0 when the connection was disconnected.

General checklist

• The commit is reviewed by yourself.
• The code is tested.
• Document is up to date or not necessary to be changed.
• The commit is compatible with repository's license.

Fix #34.