Load gltf textures correctly

[slimbuck]

This PR addresses a few issues in the gltf loader:

• multiple gltfTextures can reference a single gltfImage, each gltfTexture having it's own sampler state. When loading an image multiple times, the asset system cache kicks in an all texture assets end up sharing the same underlying pc.Texture instance. This resulted in the failed test https://cx20.github.io/gltf-test/examples/playcanvas/index.html?category=tutorialModels&model=TextureSettingsTest&scale=0.2&type=glTF. We fix this here by implementing cloneTextureAsset and cloneTexture which we use when multiple textures are found using the same underlying image.
• generate normals when position data isn't tightly packed. This is needed when position data is interleaved or when position data is quantised to short or byte (where position stride !== size due to 4-byte alignment).
• simplified of vertex semantic map handling

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