Update samplers in shaders

kitty/shaders/custom/pipeline.slang

@@ -57,12 +57,12 @@ float3 linear2srgb(float3 x) {  // vector
     return lerp(lower, upper, step(float3(0.0031308f), x));
 }
 
-uniform Sampler2D backbuffer;
+uniform Sampler2D backbuffer, a, b, persist;
 
 public float4 pipeline_fragment_main(float2 backbuffer_pos, int group) {
     float4 color = backbuffer.Sample(backbuffer_pos);  // pre multiplied linear RGB
     color = float4(color.rgb / color.a, color.a);  // un pre multiplied color passed through pipeline
-    KittyTextures t = {backbuffer, backbuffer_pos};
+    KittyTextures t = {backbuffer, a, b, persist, backbuffer_pos};
     // PIPELINE
     return vec4_premul(color.rgb, color.a);
 }

kitty/shaders/custom/types.slang

@@ -13,8 +13,15 @@ public struct KittyCustomShaderData {
 
 public struct KittyTextures {
     // the buffer containing the original rendered pixels from kitty's own rendering. This will
-    // be updated with the output color only after all custom shaders in the chain have run
+    // be updated with the output color only after all custom shaders in this group have run
     public Sampler2D backbuffer;
+    // The following are named textures that can be sampled from in custom
+    // shaders. The persist texture will persist across frames, anything
+    // written to it in render number N can be read from it in subsequent
+    // renders.
+    public Sampler2D a;
+    public Sampler2D b;
+    public Sampler2D persist;
 
     public float2 pos;  // the position of this pixel in the textures
 }