sub/osd: default to a semitransparent black back color

By default OSD and subs have no shadow, but if you do set a
--osd/sub-shadow-offset, the current default white shadow color looks
bad around white text with black border. Default to semitransparent
black instead. Use the same color box profiles were using to allow
removing it from there.

DOCS/interface-changes/back-color.txt

@@ -0,0 +1 @@
+change `--osd-back-color` and `--sub-back-color` defaults to #AF000000

etc/builtin.conf

@@ -79,7 +79,6 @@ zimg-scaler=bilinear
 zimg-dither=no
 
 [osd-box]
-osd-back-color="#AF000000"
 osd-border-style=background-box
 osd-outline-size=0
 osd-shadow-offset=4
@@ -89,7 +88,6 @@ script-opt=stats-shadow_x_offset=4
 script-opt=stats-shadow_y_offset=4
 
 [sub-box]
-sub-back-color="#AF000000"
 sub-border-style=background-box
 sub-outline-size=0
 sub-shadow-offset=4

sub/osd.c

@@ -84,7 +84,7 @@ const struct m_sub_options osd_style_conf = {
         .font_size = 30,
         .color = {255, 255, 255, 255},
         .outline_color = {0, 0, 0, 255},
-        .back_color = {240, 240, 240, 128},
+        .back_color = {0, 0, 0, 175},
         .border_style = 1,
         .outline_size = 1.65,
         .shadow_offset = 0,
@@ -104,7 +104,7 @@ const struct m_sub_options sub_style_conf = {
         .font_size = 38,
         .color = {255, 255, 255, 255},
         .outline_color = {0, 0, 0, 255},
-        .back_color = {240, 240, 240, 128},
+        .back_color = {0, 0, 0, 175},
         .border_style = 1,
         .outline_size = 1.65,
         .shadow_offset = 0,