video: new video capture architecture

[alvieboy]

In order to support software which modifies attributes and pixels synchronised with the ULA rasterization process, a few changes need to be implemented to fully capture the required data:

• Generate an internal ULA pixel clock of 7MHz / 7.0938MHz so we can "track" what ULA is rendering
• "raster" the internal framebuffer data similarly as ULA does to a second framebuffer
• Expose this new framebuffer to the system with per pixel-line attributes.