BLF: assert alpha doesn't exceed 1.0 for icon outlines

Causes undefined behavior, so the caller needs to clamp.

source/blender/blenfont/intern/blf_font.cc

@@ -526,6 +526,7 @@ void blf_draw_svg_icon(FontBLF *font,
                        bool multicolor,
                        blender::FunctionRef<void(std::string &)> edit_source_cb)
 {
+  BLI_assert(outline_alpha <= 1.0f); /* Higher values overflow, caller must ensure. */
   blf_font_size(font, size);
   font->pos[0] = int(x);
   font->pos[1] = int(y);

source/blender/blenfont/intern/blf_internal.hh

@@ -101,6 +101,9 @@ bool blf_font_size(FontBLF *font, float size);
 void blf_font_draw(FontBLF *font, const char *str, size_t str_len, ResultBLF *r_info);
 void blf_font_draw__wrap(FontBLF *font, const char *str, size_t str_len, ResultBLF *r_info);
 
+/**
+ * \param outline_alpha: Alpha value between 0 and 1.
+ */
 void blf_draw_svg_icon(FontBLF *font,
                        uint icon_id,
                        float x,