floats: fix 'winblend' on top of doublewidth chars.

The interaction between 'winblend' and doublewidth chars in the background
does not look very good. But check no chars get incorrectly placed
at least.

Also check that hidden EndOfBuffer region (from style="minimal") blends
correctly.

src/nvim/highlight_defs.h

@@ -36,6 +36,7 @@ typedef struct attr_entry {
   .rgb_sp_color = -1, \
   .cterm_fg_color = 0, \
   .cterm_bg_color = 0, \
+  .hl_blend = -1, \
 }
 
 /// Values for index in highlight_attr[].