CubeMap support

[luboslenco]

Adds support for handling cubemaps. As usual, took a bit longer than anticipated.

• CubeMap class works exactly like Image class
• WebGL is ready, will also do Kore/Krom
• Tested & working, some PR typos or minor issues can arise though
• Only render target functionality implemented so far

// Creating a cubemap
var cubemap = CubeMap.createRenderTarget(...);

// Render to cubemap using layered rendering
// Draw to all faces at once using geometry shader
// WebGL is not capable of this
cubemap.g4.begin();

// Render to desired face
// 0: x+, 1: x-, 2: y+, 3: y-, 4: z+, 5: z-
cubemap.g4.beginFace(face);

// Attach cubemap to samplerCube unit
g4.setCubeMap(cubemap);

// Attach cubemap depth buffer to samplerCube unit
g4.setCubeMapDepth(cubemap);