Theme: Set 3D Viewport background to single color

The default vignette effect introduces issues reported several times
on various hardware configurations.

It would still be nice to figure out the issue and fix it in the
vignette effect code. For now, simply use single-color in the
default theme.

As agreed during the 2023-08-15 User Interface Module meeting.

Fixes #93627
Closes #106699

release/datafiles/userdef/userdef_default_theme.c

@@ -287,7 +287,6 @@ const bTheme U_theme_default = {
   .space_view3d = {
     .back = RGBA(0x3d3d3dff),
     .back_grad = RGBA(0x30303000),
-    .background_type = 2,
     .title = RGBA(0xeeeeeeff),
     .text = RGBA(0xe6e6e6ff),
     .text_hi = RGBA(0xffffffff),
@@ -569,11 +568,11 @@ const bTheme U_theme_default = {
     .vertex_size = 3,
     .outline_width = 1,
     .facedot_size = 4,
+    .simulated_frames = RGBA(0x721e65ff),
     .keyframe_scale_fac = 1.0f,
     .handle_vertex_size = 4,
     .anim_active = RGBA(0x4d272766),
     .anim_preview_range = RGBA(0xa14d0066),
-    .simulated_frames = RGBA(0x721e65ff),
   },
   .space_nla = {
     .back = RGBA(0x30303000),