Use native GPU upload for OpenGL and Vulkan

[atteneder]

KtxUnity uploads textures to the GPU via Texture2D.LoadRawTextureData, which works on all platforms.

The underlying libktx also offers functions ktxTexture_GLUpload and ktxTexture_VkUploadEx to upload directly.

While this is more effort to implement and maintain, it has some potential upsides:

• Faster, since mipmaps don't need to get re-ordered
• Work around potential Unity format limitations (see #39)