add option to select alternative alpha blending

I'm not sure if this is needed now that we have a single draw call, but
based on the history and the nuance of different gl/driver/os quirks it
feels like a good idea to keep this option in the back pocket.

config/src/lib.rs

@@ -1195,6 +1195,9 @@ pub struct Config {
 
     #[serde(default)]
     pub pane_focus_follows_mouse: bool,
+
+    #[serde(default)]
+    pub use_alternative_alpha_blending: bool,
 }
 impl_lua_conversion!(Config);
 

wezterm-gui/src/termwindow/render.rs

@@ -543,8 +543,11 @@ impl super::TermWindow {
                 atlas_linear_sampler:  atlas_linear_sampler,
                 foreground_text_hsb: foreground_text_hsb,
             },
-            //&blend_but_set_alpha_to_one,
-            &alpha_blending,
+            if self.config.use_alternative_alpha_blending {
+                &blend_but_set_alpha_to_one
+            } else {
+                &alpha_blending
+            },
         )?;
 
         vb.next_index();