macOS: Add texture support

[jnschulze]

[WIP] This PR adds macOS texture support.

The color components are mixed up as RGBA is not supported.

In order to make this work correctly, a few things need to be fixed in the common layer (I will take care of that):

• The pixel format must be configurable by the plugin.
• The pixel buffer VLC writes to shouldn't be owned by the player but by the respective plugin. The reason is that otherwise an unnecessary memcpy operation will be required on macOS in order to turn the raw pixel buffer into a CVPixelBuffer.
• The Player implementation doesn't currently support choosing other VLC rendering backends like libvlc_video_engine_d3d11 (which will be required on Windows later on in order to support GPUSurfaceTextures). So there needs to be more flexibility in the rendering part of the Player in general.
• The platform-specific VideoOutlets should be owned by the player which will allow the macOS and Windows plugins to get rid of the outlet map.

[alexmercerind]

Should we just hardcode "RV32" here for some macOS specific macro (#ifdef)?

I believe no guy ever will want to change the video format. Still upto you, if you want to do this.

dart_vlc/dartvlc/internal/events.h

Line 155 in b27eac9

vlc_media_player_.setVideoFormat("RGBA", video_width, video_height,

dart_vlc/dartvlc/internal/events.h

Line 147 in b27eac9

strcpy(chroma, "RGBA");

[jnschulze]

Should we just hardcode "RV32" here for some macOS specific macro (#ifdef)?

I believe no guy ever will want to change the video format. Still upto you, if you want to do this.

dart_vlc/dartvlc/internal/events.h

Line 155 in b27eac9

vlc_media_player_.setVideoFormat("RGBA", video_width, video_height,

dart_vlc/dartvlc/internal/events.h

Line 147 in b27eac9

strcpy(chroma, "RGBA");

Yes, this should be fine as a quick fix.

Btw., this strcpy causes a warning for me. Just use memcpy(chroma, "RGBA", 4) instead.

(In general, I'd like to decouple video output from the player and abstract it in a way that the platform-specific plugin can create the most suitable output itself. I Started working on that on Monday.)