all: add a compiler directive kage:unit

This change adds a new compiler directive 'kage:unit' to Kage. This
takes one of these two values: 'pixel' and 'texel'. The default value
is 'texel'.

With the pixel-unit mode, all the built-in functions treats pixels
instead of texels, and the texCoord argument of Fragment is in pixels.
This simplifies shader programs as programs no longer have the notion
of texels.

With the texel-unit mode, the behavior is the same as the current
behavior.

Closes #1431

examples/shader/dissolve.go

@@ -14,11 +14,12 @@
 
 //go:build ignore
 
+//kage:unit pixel
+
 package main
 
 var Time float
 var Cursor vec2
-var ScreenSize vec2
 
 func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 	// Triangle wave to go 0-->1-->0...