metal: Misc. improvements.

- Use a single buffer for various non-changing constants accessed by the GPU, instead of multiple buffers.
- Do the half-pixel offset for points and lines using a transform matrix so we don't need a malloc when rendering.
- Don't add a half-pixel offset for other primitives and textures. This matches D3D and GL render behaviour.
- Remove the half-texel texture coordinate offset since it's not needed now that there's no more half-pixel position offset when rendering a texture.
- Don't try to set texture usage on iOS 8 since it doesn't exist there.

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