Add cube map support to engine

[simon-bourque]

TASK

Description:

We need to be able to create and load cube maps. Both the converter and TextureCache need to be updated to support the new texture type. Add the new type the the Target enum in Texture.h and then add the new case the loadTexture function. Base yourself of the implementation of the current texture types. For the converter also base yourself off what is already there.

Motivation:

Need cube map textures for skyboxes. It is currently blocking us from replacing the old texture manager with the new TextureCache for the 3D engine.

Completion Criteria:

• Cube map type added to Target enum
• Can load cube maps using the TextureCache
• You can convert a set of images into a cube map using our converter
• Cube map should be saved using our current file format