Add X-Ray view mode to 3D viewport (#15770)

resources/shaders/140/xray.fs

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+#version 140
+
+// X-Ray view: each surface is a thin translucent layer of its own color, covering more the more
+// edge-on it is seen. The caller draws it with depth writes off, so every surface a view ray crosses
+// composites and overlapping geometry reads as denser. Source-alpha rather than additive blending:
+// additive would saturate to white on the light theme's near-white background.
+#define EDGE_FALLOFF 2.0
+// Coverage head-on and edge-on.
+#define FACE_DENSITY 0.12
+#define EDGE_DENSITY 0.65
+
+uniform vec4 uniform_color;
+
+in vec3 eye_normal;
+in vec3 eye_position;
+in vec3 clipping_planes_dots;
+
+out vec4 out_color;
+
+void main()
+{
+    if (any(lessThan(clipping_planes_dots, vec3(0.0))))
+        discard;
+
+    // abs(): back faces are drawn too, and there only the angle matters.
+    float facing = abs(dot(normalize(eye_normal), normalize(-eye_position)));
+    float density = mix(EDGE_DENSITY, FACE_DENSITY, pow(facing, EDGE_FALLOFF));
+
+    out_color = vec4(uniform_color.rgb, density * uniform_color.a);
+}

resources/shaders/140/xray.vs

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+#version 140
+
+uniform mat4 view_model_matrix;
+uniform mat4 projection_matrix;
+uniform mat3 view_normal_matrix;
+uniform mat4 volume_world_matrix;
+
+// Clipping, as in gouraud.vs: the preview's top / bottom plane and the SLA gizmo's free one.
+uniform vec2 z_range;
+uniform vec4 clipping_plane;
+
+in vec3 v_position;
+in vec3 v_normal;
+
+out vec3 eye_normal;
+out vec3 eye_position;
+out vec3 clipping_planes_dots;
+
+void main()
+{
+    eye_normal = view_normal_matrix * v_normal;
+
+    vec4 position = view_model_matrix * vec4(v_position, 1.0);
+    eye_position = position.xyz;
+
+    // A component below zero means the fragment is clipped away.
+    vec4 world_pos = volume_world_matrix * vec4(v_position, 1.0);
+    clipping_planes_dots = vec3(dot(world_pos, clipping_plane), world_pos.z - z_range.x, z_range.y - world_pos.z);
+
+    gl_Position = projection_matrix * position;
+}