Add additional debug output when enabled

Print compilation and linking shader log despite the compilation and
linking statuses. This is useful at least on NVidia cards where driver
also provides warning messages.
Update texture creation

Use glTexStorage2D for storage allocation, allocate 1 mipmap for depth
and color fb attachements at most.
Allocate 8 mipmap textures for generic image textures.
Do not call glGenerateMipmap for fb color textures since no mipmap levels
are defined and the call does nothing.