Draw glass behind translucent panels on 1.21.11

A copy of the game behind the screen is blurred once a frame and drawn
through each panel's rounded rectangle texture by a glass pipeline.
Older versions keep blurring the whole background.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/main/resources/assets/saturnclient/shaders/core/glass.fsh

@@ -0,0 +1,31 @@
+#version 330
+
+// Saturn's glass: the blurred game behind the screen, cut to a panel's shape.
+
+layout(std140) uniform DynamicTransforms {
+    mat4 ModelViewMat;
+    vec4 ColorModulator;
+    vec3 ModelOffset;
+    mat4 TextureMat;
+};
+
+// The panel's rounded rectangle, the same texture its background is drawn with
+uniform sampler2D Sampler0;
+// The game behind the screen, blurred, the size of the framebuffer
+uniform sampler2D Sampler1;
+
+in vec2 texCoord0;
+in vec4 vertexColor;
+
+out vec4 fragColor;
+
+void main() {
+    float mask = texture(Sampler0, texCoord0).a * vertexColor.a;
+    if (mask == 0.0) {
+        discard;
+    }
+
+    // The blurred copy lines up with the framebuffer being drawn to.
+    vec2 screenUv = gl_FragCoord.xy / vec2(textureSize(Sampler1, 0));
+    fragColor = vec4(texture(Sampler1, screenUv).rgb, mask) * ColorModulator;
+}