Fill only the area that was modified by the previous GIF frame

[TWiStErRob]

Description

As seen in #1059:

If you look closely you can see there are multiple different flickers:

• one where the frame is partially shown
• one where a single frame is almost fully transparent

This is how it should look like:

This change achieves that by clearing only part of the frame when need to dispose to background.

Motivation and Context

I sent the above image through http://ezgif.com/split ("Ignore optimizations") to see what how they look:

As you can see there's an actual half-frame and there are many almost-empty frames, most importantly the one where there's a little pink on the left. These two frames cause the trouble. Their dispose is set to "background" which triggers a code block that fills the entire frame with bgColor. The problem is that the frame doesn't contain a full-sized image, just a partial one. This is also confirmed in debug (closed frames look identical to 0th frame):

I checked the GIF spec and it says:

iv) Disposal Method - Indicates the way in which the graphic is to be treated after being displayed.
2 - Restore to background color. The area used by the graphic must be restored to the background color. -- GIF89a, emphasis mine

This means that when preparing the frame transition from 2 to 3, 2 wants it's partial area to be cleared as defined by the (ix,iy iw,ih) rect. I took a shot at implementing that, I went through these iterations to arrive at the code in the PR.

1. simple iteration with cached end values

   for (int y = previousFrame.iy, yEnd = y + previousFrame.ih; y < yEnd; y++) {
       for (int x = previousFrame.ix, xEnd = x + previousFrame.iw; x < xEnd; x++) {
           dest[y * width + x] = c;
       }
   }

2. pre-calculate values and name them

   int top = previousFrame.iy;
   int bottom = previousFrame.iy + previousFrame.ih;
   int left = previousFrame.ix;
   int right = previousFrame.ix + previousFrame.iw;
   for (int y = top; y < bottom; y++) {
       for (int x = left; x < right; x++) {
           dest[y * width + x] = c;
       }
   }

3. lift multiplication up

   int top = previousFrame.iy;
   int bottom = previousFrame.iy + previousFrame.ih;
   for (int y = top; y < bottom; y++) {
       int left = y * width + previousFrame.ix;
       int right = y * width + previousFrame.ix + previousFrame.iw;
       for (int p = left; p < right; p++) {
           dest[p] = c;
       }
   }

4. lift multiplication up again and rename variables

   int topLeft = previousFrame.iy * width + previousFrame.ix;
   int bottomLeft = topLeft + previousFrame.ih * width;
   for (int left = topLeft; left < bottomLeft; left += width) {
       int right = left + previousFrame.iw;
       for (int pointer = left; pointer < right; pointer++) {
           dest[pointer] = c;
       }
   }

Not sure which one is best, I think the final version is readable enough and all multiplications are lifted outside, so it should be efficient as well.

[TWiStErRob]

/cc @kojilin

[coveralls]

Coverage remained the same at 83.158% when pulling e15172e on TWiStErRob:gif_dispose into e68db00 on bumptech:3.0.

[sjudd]

LGTM, awesome work!