render: split rendering into two phases

In the first phase we collect a list of simple operations (copying
textures and filling rectangles.)

In the second phase we send this list to the graphics API to be
executed.

As part of this, we also remove the use of scissors.

src/theme.rs

@@ -1,6 +1,9 @@
-use std::cell::{Cell, RefCell};
+use std::{
+    cell::{Cell, RefCell},
+    cmp::Ordering,
+};
 
-#[derive(Copy, Clone, Debug)]
+#[derive(Copy, Clone, Debug, PartialEq)]
 pub struct Color {
     pub r: f32,
     pub g: f32,
@@ -8,6 +11,24 @@ pub struct Color {
     pub a: f32,
 }
 
+impl Eq for Color {}
+
+impl Ord for Color {
+    fn cmp(&self, other: &Self) -> Ordering {
+        self.r
+            .total_cmp(&other.r)
+            .then_with(|| self.g.total_cmp(&other.g))
+            .then_with(|| self.b.total_cmp(&other.b))
+            .then_with(|| self.a.total_cmp(&other.a))
+    }
+}
+
+impl PartialOrd for Color {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
 fn to_f32(c: u8) -> f32 {
     c as f32 / 255f32
 }