Prepare of alpha blending of cursor

Will allow for cursor blink animation eventually.

kitty/data-types.h

@@ -300,9 +300,10 @@ typedef struct {
 } Cursor;
 
 typedef struct {
-    bool is_visible, is_focused, render_even_when_unfocused;
+    bool is_focused, render_even_when_unfocused;
     CursorShape shape;
     unsigned int x, y;
+    float opacity;
 } CursorRenderInfo;
 
 typedef enum DynamicColorType {