EEVEE-Next: HiZ Buffer: New implementation

This new implementation does all downsampling in a single compute shader
dispatch, removing a lot of complexity from the previous recursive
downsampling.

This is heavilly inspired by the Single-Pass-Downsampler from GPUOpen:
https://github.com/GPUOpen-Effects/FidelityFX-SPD
However I do not implement all the optimization bits as they require
vulkan (GL_KHR_shader_subgroup) and is not as versatile (it is only
for HiZ).

Timers inside renderdoc report ~0.4ms of saving on a 2048*1024 render for
the whole downsampling. Note that the previous implementation only
processed 6 mips where the new one processes 8 mips.
```
EEVEE ~1.0ms
EEVEE-Next ~0.6ms
```

Padding has been bumped to be of 128px for processing 8 mips.

A new debug option has been added (debug value 2) to validate the HiZ.

1 files changed, 24 insertions, 0 deletions

diff --git a/source/blender/draw/engines/eevee_next/shaders/eevee_hiz_debug_frag.glsl b/source/blender/draw/engines/eevee_next/shaders/eevee_hiz_debug_frag.glsl
new file mode 100644
index 00000000000..e93d0f472fa
--- /dev/null
+++ b/source/blender/draw/engines/eevee_next/shaders/eevee_hiz_debug_frag.glsl
@@ -0,0 +1,24 @@
+
+/**
+ * Debug hiz down sampling pass.
+ * Output red if above any max pixels, blue otherwise.
+ */
+
+void main()
+{
+  ivec2 texel = ivec2(gl_FragCoord.xy);
+
+  float depth0 = texelFetch(hiz_tx, texel, 0).r;
+
+  vec4 color = vec4(0.1, 0.1, 1.0, 1.0);
+  for (int i = 1; i < HIZ_MIP_COUNT; i++) {
+    ivec2 lvl_texel = texel / ivec2(uvec2(1) << uint(i));
+    lvl_texel = min(lvl_texel, textureSize(hiz_tx, i) - 1);
+    if (texelFetch(hiz_tx, lvl_texel, i).r < depth0) {
+      color = vec4(1.0, 0.1, 0.1, 1.0);
+      break;
+    }
+  }
+  out_debug_color_add = vec4(color.rgb, 0.0) * 0.2;
+  out_debug_color_mul = color;
+}