add edge drawing, stereographic geometry shader for handling curved projections

vis/shaders/stereo-proper.gm.glsl

@@ -0,0 +1,25 @@
+#version 430
+
+#define SUBS 20
+
+layout(lines) in;
+layout(line_strip, max_vertices = SUBS) out;
+
+layout(location=0) uniform mat4 proj;
+
+in vec4 gpos[];
+out vec4 vpos;
+
+vec4 stereo(vec4 v) {
+    return vec4(v.xyz / (1 - v.w), 1);
+}
+
+void main() {
+    for (int i = 0; i < SUBS; i++) {
+        vpos = mix(gpos[0], gpos[1], i * 1.0f / (SUBS - 1));
+        vpos = normalize(vpos);
+        gl_Position = proj * stereo(vpos);
+        EmitVertex();
+    }
+    EndPrimitive();
+}