sokol: don't draw from offscreen texture at all; merge shaders

we don't want multisampling when drawing to the picking texture,
but we can't simultaneously draw with multisampling to another
texture, so instead just don't do anything BUT drawing to the
picking texture, before doing the rest anyway. with msaa
enabled, this reduces performance somewhat, but is still much
smoother and avoids both all the complication and drawing
issues.

- up to date sokol-shdc output, consolidate edge/node shaders
  into the same module, avoiding redundant uniform declarations

glsl/edge.glsl

@@ -1,4 +1,4 @@
-@module e
+@module edge
 
 @vs vs
 @glsl_options fixup_clipspace
@@ -21,11 +21,40 @@ precision mediump float;
 layout(binding=1) uniform Fparam {
 	vec4 color;
 };
-out vec4 fc;
+out vec4 c;
 
 void main(){
-	fc = color;
+	c = color;
 }
 @end
 
-@program edge vs fs
+@vs vsoff
+@glsl_options fixup_clipspace
+
+precision mediump float;
+
+layout(binding=0) uniform Vparam {
+	mat4 mvp;
+};
+in vec3 pos;
+flat out uint idx;
+
+void main(){
+	gl_Position = mvp * vec4(pos, 1.0);
+	idx = (gl_VertexIndex >> 1) + 1 | 1<<31;
+}
+@end
+
+@fs fsoff
+precision mediump float;
+
+flat in uint idx;
+out uint fc;
+
+void main(){
+	fc = idx;
+}
+@end
+
+@program idx vsoff fsoff
+@program s vs fs