Metal: enable mipmap support

[smilediver]

Enables support for mipmapped textures in Metal. In Metal, mipmaps were ignored, because mipFilter for MTLSamplerDescriptor was never set.

Note: new graphic APIs like Metal do not support separate mipmap filters for minifying and magnifying textures, so I had to make a compromise and derive mip filter from the mag filter. I think in the future it would be best to change SamplerFilter enum to contain only LINEAR and NEAREST values and add SamplerMipFilter enum to contain LINEAR, NEAREST, NONE and in SamplerDescriptor add SamplerMipFilter mipFilter field.