UI: Improved overlay text contrast with new outline text decoration

Overlay texts were previously drawn with two sets of shadows:
- 3px blur,
- 5px blur, slightly offset

But since the shadow color was always set to black, it was still
causing legibility issues when the text itself was dark (set
via theme for example).

This PR adds a new "outline" BLF text decoration, and uses that
for the overlays. And it picks text/outline color depending
on the "background" color of the view.

Details:

- Instead of "shadow level" integer where the only valid options
  are 0, 3 or 5, have a FontShadowType enum.
- Add a new FontShadowType::Outline enum entry, that does a 1px
  outline by doing a 3x3 dilation in the font shader.
- BLF_draw_default_shadowed is changed to do outline, instead of
  drawing the shadow twice.
- In the font shader, instead of encoding shadow type in signs of
  the glyph_size, pass that as a "flags" vertex attribute. Put
  font texture channel count into the same flags, so that the
  vertex size stays the same.
- Well actually, vertex size becomes smaller by 4 bytes, since turns
  out glyph_mode vertex attribute was not used for anything at all.

Images in the PR.

Co-authored-by: Harley Acheson <harley.acheson@gmail.com>
Pull Request: https://projects.blender.org/blender/blender/pulls/121383

source/blender/python/generic/blf_py_api.cc

@@ -342,7 +342,7 @@ PyDoc_STRVAR(
     "   :arg fontid: The id of the typeface as returned by :func:`blf.load`, for default "
     "font use 0.\n"
     "   :type fontid: int\n"
-    "   :arg level: The blur level, can be 3, 5 or 0.\n"
+    "   :arg level: The blur level (0, 3, 5) or outline (6).\n"
     "   :type level: int\n"
     "   :arg r: Shadow color (red channel 0.0 - 1.0).\n"
     "   :type r: float\n"
@@ -363,12 +363,12 @@ static PyObject *py_blf_shadow(PyObject * /*self*/, PyObject *args)
     return nullptr;
   }
 
-  if (!ELEM(level, 0, 3, 5)) {
-    PyErr_SetString(PyExc_TypeError, "blf.shadow expected arg to be in (0, 3, 5)");
+  if (!ELEM(level, 0, 3, 5, 6)) {
+    PyErr_SetString(PyExc_TypeError, "blf.shadow expected arg to be in (0, 3, 5, 6)");
     return nullptr;
   }
 
-  BLF_shadow(fontid, level, rgba);
+  BLF_shadow(fontid, FontShadowType(level), rgba);
 
   Py_RETURN_NONE;
 }