Support mixing the two HDMI inputs together

[mithro]

Some example modes would be;

• Mask mode - This is where a mask is provided which selects which input each pixel comes from.
• Overlay mode - This is where any pixel with a given color value is replaced with the pixel from the other input.
• Others?

[enjoy-digital]

some stuff here: https://github.com/m-labs/mixxeo-soc/tree/master/mixxeolib

[mithro]

Start of a design document around this stuff is at https://docs.google.com/document/d/1ZjM1Brrks0lg1CJp2Rt1BH8-MhJamrKUeCUB4s4nzoA/edit#heading=h.kttxlrugap8z

[mithro]

@ssk1328 - Take a look at this issue.