image_writer: make JPG default and increase quality (for screenshots)

The assumption is that JPG screenshots are more useful in general.
Lossless screenshots made from lossy videos are just a waste of space.

Increase JPEG quality a bit. There's a tradeoff between quality and
size, and since JPEG is the default now, attempt to balance the
JPEG settings to provide sane defaults for general use cases.

image_writer.c

@@ -46,9 +46,9 @@
 #include "m_option.h"
 
 const struct image_writer_opts image_writer_opts_defaults = {
-    .format = "png",
+    .format = "jpg",
     .png_compression = 7,
-    .jpeg_quality = 85,
+    .jpeg_quality = 90,
     .jpeg_optimize = 100,
     .jpeg_smooth = 0,
     .jpeg_dpi = 72,