Add a clear_color uniform

Introduced an RGBA8 config parameter to apply as a base blend color in
the fine stage of the full pipeline.

shader/fine.wgsl

@@ -187,6 +187,9 @@ fn main(
     let xy = vec2(f32(global_id.x * PIXELS_PER_THREAD), f32(global_id.y));
 #ifdef full
     var rgba: array<vec4<f32>, PIXELS_PER_THREAD>;
+    for (var i = 0u; i < PIXELS_PER_THREAD; i += 1u) {
+        rgba[i] = unpack4x8unorm(config.clear_color).wzyx;
+    }
     var blend_stack: array<array<u32, PIXELS_PER_THREAD>, BLEND_STACK_SPLIT>;
     var clip_depth = 0u;
     var area: array<f32, PIXELS_PER_THREAD>;

shader/shared/config.wgsl

@@ -7,6 +7,11 @@ struct Config {
     target_width: u32,
     target_height: u32,
 
+    // The initial color applied to the pixels in a tile during the fine stage.
+    // This is only used in the full pipeline. The format is packed RGBA8 in MSB
+    // order.
+    clear_color: u32,
+
     n_drawobj: u32,
     n_path: u32,
     n_clip: u32,

src/encoding/packed.rs

@@ -60,6 +60,8 @@ pub struct Config {
     pub target_width: u32,
     /// Height of the target in pixels.
     pub target_height: u32,
+    /// The initial background color applied to the target
+    pub clear_color: u32,
     /// Layout of packed scene data.
     pub layout: Layout,
     /// Size of binning buffer allocation (in u32s).

src/render.rs

@@ -5,6 +5,7 @@ use bytemuck::{Pod, Zeroable};
 use crate::{
     encoding::Encoding,
     engine::{BufProxy, ImageFormat, ImageProxy, Recording, ResourceProxy},
+    peniko::Color,
     shaders::{self, FullShaders, Shaders},
     Scene,
 };
@@ -21,6 +22,8 @@ pub struct Render {
     ptcl_size: u32,
     width_in_tiles: u32,
     height_in_tiles: u32,
+    /// The background color applied to the target
+    clear_color: Color,
     fine: Option<FineResources>,
 }
 
@@ -61,6 +64,7 @@ struct Config {
     height_in_tiles: u32,
     target_width: u32,
     target_height: u32,
+    clear_color: u32,
     n_drawobj: u32,
     n_path: u32,
     n_clip: u32,
@@ -216,6 +220,7 @@ impl Render {
             ptcl_size: (1 << 25) / 4 as u32,
             width_in_tiles: 0,
             height_in_tiles: 0,
+            clear_color: Color::BLACK,
             fine: None,
         }
     }
@@ -265,6 +270,7 @@ impl Render {
             height_in_tiles: new_height / 16,
             target_width: width,
             target_height: height,
+            clear_color: self.clear_color.to_premul_u32(),
             binning_size: self.binning_info_size - info_size,
             tiles_size: self.tiles_size,
             segments_size: self.segments_size,