gpu: implement automatic mipmaps for images

All GPU APIs except OpenGL ES 2 can generate mipmaps for textures.
This trades 33% more GPU memory use for improved rendering quality
and speed for downscaled images.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/internal/driver/driver.go

@@ -166,6 +166,7 @@ const (
 const (
 	FilterNearest TextureFilter = iota
 	FilterLinear
+	FilterLinearMipmapLinear
 )
 
 const (