Texture can be constructed with a specified number of mip levels

[mvaligursky]

• texture could have been be created with options.mipmaps, which internally allocated a full mip map chain
• now options.mipLevelCount can be specified, which limits the mipmaps to specified number - useful when you need just one of two additional mip levels.
• we used to not allow mipmaps for integer texture formats, even though it's a valid use case. This is now allowed, but automatic mipmap generation for those texture is disablef. The user can manually render to the levels though.