Improve performance of mipmaps creation for some texture formats

[CedricGuillemet]

Related : #1391

For some texture formats are converted to RGBA8 or RGBA32F before computing mipmaps because of bgfx limitations. Add support for most(all?) texture formats.

see

BabylonNative/Plugins/NativeEngine/Source/NativeEngine.cpp

Line 214 in efb8fb2

if (image->m_format == bimg::TextureFormat::RGB8)

[bghgary]

No one has asked for this, we noticed this while adding some formats. We can improve memory usage and possibly CPU cost if we do this work.

[bghgary]

Closing for now. Reopen if we want to fix this.