Fix external outline Thickness (#14741)

* Fix external outline

* Format and values

* frag_color -> gl_FragColor

* Remove Transform3d& view_matrix

* Clarify rendering comments in 3DScene.cpp

Updated comments to clarify rendering process using stencil buffer.

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Co-authored-by: Ian Bassi <ian.bassi@outlook.com>
Co-authored-by: Noisyfox <timemanager.rick@gmail.com>

resources/shaders/140/ssao.fs

@@ -10,6 +10,7 @@ uniform sampler2D depth_texture;
 uniform sampler2D normal_texture;   
 uniform float z_near;
 uniform float z_far;
+uniform bool is_outline;
 
 in vec2 tex_coord;
 out vec4 frag_color;
@@ -22,6 +23,10 @@ float linearize_depth(float depth)
 
 void main()
 {
+    if (is_outline) {
+        frag_color = vec4(texture(color_texture, tex_coord).rgb, 1.0);
+        return;
+    }
     ivec2 pixel = ivec2(gl_FragCoord.xy);
     float center_depth = linearize_depth(texelFetch(depth_texture, pixel, 0).r);