GPencil: Fix clamp alpha problems when using Regular blend

The problem was the alpha was not premult and the opacity factor was applied two times.

source/blender/draw/engines/gpencil/shaders/gpencil_blend_frag.glsl

@@ -115,7 +115,11 @@ void main()
   if (mode == MODE_REGULAR) {
     if (stroke_color.a > 0) {
       if (mix_color.a > 0) {
-        FragColor = vec4(mix(stroke_color.rgb, mix_color.rgb, mix_color.a * blend_opacity),
+      /* premult */
+        stroke_color = vec4(vec3(stroke_color.rgb / stroke_color.a), stroke_color.a);
+        mix_color = vec4(vec3(mix_color.rgb / mix_color.a), mix_color.a);
+
+        FragColor = vec4(mix(stroke_color.rgb, mix_color.rgb, mix_color.a),
                          stroke_color.a);
         gl_FragDepth = mix_depth;
       }