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
With DirectX, the graphics driver cannot be determined until the
main loop starts, as a transparent window cannot be treated with
DirectX so far. On the other hand, compiling shaders requires a
graphics driver as it requires information about Y directions of
NDCs and framebuffers.
This change delays compiling shaders until the graphics commands
are actually executed in the main loop.
Updates #1007
Updates #2019
