Fix T70381: Motion Paths off by one

The apparent off-by-one error was caused by a few factors:

- The 'blend base' colour was green for the two frames directly
  surrounding the current frame, but black for the current frame itself.
- For the frames before the current one, the 'blend base' was mixed with
  black, making the green stand out clearly, but fading to black again
  for the current frame. This looks like an off-by-one, even though it
  was just bad mixing.
- For the frames after the current one, the 'blend base' was mixed with
  cyan, which already has a strong green component, so mixing it there
  was much less visible, making the entire thing look like an off-by-one
  error where it actually wasn't.

I have simplified the code, so now it only chooses green as the 'blend
base' for the current frame, and simplified the mixing for the current
frame.

source/blender/draw/engines/overlay/shaders/motion_path_line_vert.glsl

@@ -32,7 +32,7 @@ void main()
 
   float intensity; /* how faint */
 
-  vec3 blend_base = (abs(frame - frameCurrent) == 1) ?
+  vec3 blend_base = (abs(frame - frameCurrent) == 0) ?
                         colorCurrentFrame.rgb :
                         colorBackground.rgb; /* "bleed" cframe color to ease color blending */
   bool use_custom_color = customColor.x >= 0.0;
@@ -78,13 +78,12 @@ void main()
     else {
       /* green - on frameCurrent */
       if (selected) {
-        intensity = 0.5f;
+        intensity = 0.92f;
       }
       else {
-        intensity = 0.99f;
+        intensity = 0.75f;
       }
-      finalColor_geom.rgb = clamp(
-          mix(colorCurrentFrame.rgb, colorBackground.rgb, intensity) - 0.1, 0.0, 0.1);
+      finalColor_geom.rgb = mix(colorBackground.rgb, blend_base, intensity);
     }
   }