add support for multiple cameras. camera switching

[jacokotze]

Add support for multiple cameras whilst keeping a predefined camera.

Semi-breaking due to needing users to use g3d.camera.current() in order to retrieve the current camera.

Move Camera to an object, with the static component tracking the "current" active camera.

Additionally models may now specify a camera to draw with, defaulting to the current active camera.

Tested in a local project roughly :

local g3d = require('g3d')
local cameras = { "camA"=g3d.camera.current(); "camB"=g3d.camera.newCamera() };

function love.load()
 cameraUsed = "camA"
end

function love.keypressed(key, scancode)
 if key == "f" then
  if cameraUsed == "camA" then
   cameraUsed = "camB"
  else
   cameraUsed = "camA"
  end
  g3d.camera.setCurrent(cameras[cameraUsed])
 end
end

-- love.draw(); love.update() as per demo

Causes the game to switch between cameras.

Could be useful when using multiple view games (in game cctv etc)