Instead of using a negative height in the viewport, invert the Y direction
at the vertex shader. This is a little more readable as a negative height
is hacky.
This is a preparation for DirectX 12. DirectX 12's coodination system
is very similar to Metal, but doesn't treat a negative height in its
viewport unfortunately.
Updates #1007
Note that this applies only to the builtin shaders - interface for Kage stays
unchanged for compatibility.
Minor compatibility delta: when interpolating alpha values, previous code has
created nonsense values, such as, when interpolating from
fully-transparent-black (0,0,0,0) to opaque-white (1,1,1,1), something like
half-transparent-grey (0.25,0.25,0.25,0.5) where half-transparent-white
(0.5,0.5,0.5,0.5) is used by the new code.
I assume this is a strict improvement, however this may warrant some testing.
Possible later improvement could be moving the premultiplication from fragment
shader to CPU. Did not do this as it makes the code rather inconsistent of Kage
vs built-in shader usage.
Updates #1772
