vaapipostproc: don't do any color conversion when GL_TEXTURE_UPLOAD
Since gst_vaapi_find_preferred_caps_feature() returns a color format from caps negotiation, different from the default one (NV12), the postproc enables the color transformation. But when GL_TEXTURE_UPLOAD feature is negotiated, no color transformation shall be done.
Nonetheless, with commit 3e992d8a the requested format changes firstly, because there's no video sink yet, so ANY caps are negotiated; but later, when there's a video sink and a caps renegotiation, the GL_TEXTURE_UPLOAD is negotiated though the color format conversion still ongoing. It is required to reset that conversion.
This patch force default color format when GL_TEXTURE_UPLOAD is selected as preferred, thus avoiding the color conversion.
Fixes: #157 (closed)