Fix the CPU sky-shader demo.

[nnethercote]

It's supposed to produce a landscape with a small white sun, blue sky, and yellow ground. Instead it produces a small white sun and everything else is black. The problem is with the
sun_intensity_extra_spec_const_factor argument to sky_shader::fs.

• When running on the GPU, sky_shader::fs is called from main_fs, which has a spec_constant with a default value of 100.

• When running on the CPU, sky_shader::fs is called directly from the CPU shader's main, and it passes 1.

In other words, the CPU shader's sun intensity is 100x too small, which explains why it's so dark. This commit changes the value to 100, which makes the CPU shader produce the expected result.

(The shader later divides the intensity value by 100. There are comments about integration testing for specialization constants that I don't understand.)

[nnethercote]

It looks like the CPU sky shader was broken in 4c6cf0d, which added the 100x intensity value in order to implement adjustable sun brightness in the ash runner. The CPU runner wasn't updated accordingly (it needed to be because it doesn't go via main_fs, unlike the GPU runners) and I guess there are no tests for the CPU runner.

[eddyb]

My bad - I never put this up as a PR for whatever reason:

$ git show 97c0e5be
commit 97c0e5be87dab30b0ea26495845858d5adbb882c
Author: Eduard-Mihai Burtescu <[email protected]>
Date:   Sat Nov 25 09:43:57 2023 +0200

    examples/runners/cpu: fix intensity factor, and increase resolution.

diff --git a/examples/runners/cpu/src/main.rs b/examples/runners/cpu/src/main.rs
index 30c12a9420d..90dc4da78af 100644
--- a/examples/runners/cpu/src/main.rs
+++ b/examples/runners/cpu/src/main.rs
@@ -97,8 +97,8 @@ fn color_u32_from_vec4(v: Vec4) -> u32 {
 }
 
 fn main() {
-    const WIDTH: usize = 1280;
-    const HEIGHT: usize = 720;
+    const WIDTH: usize = HEIGHT * 16 / 9;
+    const HEIGHT: usize = 1080;
 
     let mut window = Window::new(
         "Rust GPU - CPU shader evaluation",
@@ -141,7 +141,7 @@ fn main() {
                 * vec2(WIDTH as f32, HEIGHT as f32);
 
             // evaluate the fragment shader for the specific pixel
-            let color = shader_module::fs(&push_constants, frag_coord, 1);
+            let color = shader_module::fs(&push_constants, frag_coord, 100);
 
             color_u32_from_vec4(color)
         })

(ironically, because I keep rebasing everything on top of main, this commit now makes no sense, because it has the resolution but not the intensity factor change)