Improve texture formats choice

[Guekka]

From Robert, Skyblivion developer on xEdit Discord :

so BC4 is same size as BC1, but you get triple the pixel data as its single channel instead of 3
and its 8-bit endpoints instead of 5-6-5 higher precision essentially
_D should be BC1 or BC7 if Transparency needed
_N should be BC1 or BC7 if Specular needed
_G should be BC1
_P should be BC4
_E should be BC1
_M should be BC4 sometimes people use _EM
_I should be BC1
_S should be BC1
_B should be BC1
_SK should be BC1

BC3 also supports an Alpha Channel and some folks use that for _Ds and _Ns though BC7 will produce better results with the tradeoff being its slower to calculate the Texel modes per block