[Rend2] Map colors from 0-255 range to 0-1 range

Rend2 stores colours as floating point numbers in the 0 - 1 range, as
opposed to the original renderer which stores colours as bytes in 0 - 255
range. This commit scales the colours down to the correct range and data
type.
[Rend2] Corrected default shader creation.

There was a small mistake when porting the vanilla renderer code to rend2
- when finding shaders, LIGHTMAP_NONE should be passed as the lightmap
  index, if a lightmap should not be used. Previously, 0 was passed,
  indicating that lightmap 0 should be used with the shader.