Save PNG compressed, WebP at effort 4 and AVIF at effort 1 (closes #232)

Each output format now has its own govips export with its settings
named, in place of govips' generic Export, which sent libvips a zero for
some settings it was not given: PNG had no compression and WebP effort
0. PNG now gets libvips' default compression, 6, and WebP its default
effort, 4. GIF was already at libvips' default effort, and JPEG output
is unchanged.

AVIF was at libvips' default effort, 4, which takes minutes for an
8192x8192 image with one libvips thread, far past the default
downstream_timeout; effort 1 takes about 12 seconds. A 16-bit source,
which libvips would save with 12 bits per sample, gets 8: at effort 1,
12 bits take about 54 seconds.

Model: opus-5-5

internal/imageprocessor/encoder_settings_internal_test.go

@@ -105,8 +105,8 @@ func TestImageProcessor_WebPAtDefaultEffort(t *testing.T) {
 // TestImageProcessor_AVIFAtEffort1 verifies that AVIF is saved at effort 1
 // with 8 bits per sample: the output is the size of the same image saved
 // with those settings. The source is 16-bit, which libvips saves with 12
-// bits when it is not given a bit depth, and 640x480: at smaller sizes,
-// such as 256x192, efforts 1 and 2 give the same output.
+// bits when it is not given a bit depth, and 640x480: on a small image,
+// such as 64x48, efforts 1 and 2 give the same output.
 func TestImageProcessor_AVIFAtEffort1(t *testing.T) {
 	t.Parallel()