Add epaint::Brush for controlling RectShape texturing (#5565)

Also wraps `Shape::Mesh` in an `Arc`.

No new features, but decreases size of `Shape` from 72 bytes to 64.

crates/epaint/src/brush.rs

@@ -0,0 +1,19 @@
+use crate::{Rect, TextureId};
+
+/// Controls texturing of a [`crate::RectShape`].
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+pub struct Brush {
+    /// If the rect should be filled with a texture, which one?
+    ///
+    /// The texture is multiplied with [`crate::RectShape::fill`].
+    pub fill_texture_id: TextureId,
+
+    /// What UV coordinates to use for the texture?
+    ///
+    /// To display a texture, set [`Self::fill_texture_id`],
+    /// and set this to `Rect::from_min_max(pos2(0.0, 0.0), pos2(1.0, 1.0))`.
+    ///
+    /// Use [`Rect::ZERO`] to turn off texturing.
+    pub uv: Rect,
+}