Fixed calcNumMips for non-power-of-2 texture dimensions

[jay3d]

calcNumMips function was calculating often wrong number of mipmaps for textures whose dimensions are not power of 2 such as 96x96; Apparently this is not an issue in Vulkan and probably others(IMHO is bad), but Metal seems to complain about it:

-[MTLTextureDescriptorInternal validateWithDevice:]:1357: failed assertion `Texture Descriptor Validation
MTLTextureDescriptor requests 8 mipmap levels, but the dimensions (96, 96, 1) can only support a maxiumum of 7 levels
'

I introduced floorLog2 function in bx so now this function can use it instead of ceilLog2, which works properly for all texture dimensions.

PS: This PR might fail checks because it might not be using the updated bx: bkaradzic/bx#314