fix(gif): Preserve RGB values of transparent pixels (#5188)

Preserve RGB of transparent color index pixels in GIF files.

When reading GIF files, pixels of transparent color index were skipped.
This left their RGB channels zeroed, discarding possible valid colormap
data. To preserve the RGB values, we change to always write the
colormap's RGB and set alpha conditionally.

According to [GIF spec](https://www.w3.org/Graphics/GIF/spec-gif89a.txt):

>viii) Transparency Index - The Transparency Index is such that when
encountered, the corresponding pixel of the display device is not
modified and processing goes on to the next pixel. The index is present
if and only if the Transparency Flag is set to 1.

So basically, it just defines a display compositing behavior but says
nothing about the color table entry for that index being invalid or
empty. In practice, encoders may still assign meaningful RGB values to
the transparent index, and those values are perfectly well-defined in
the colormap. For example, GIMP reads the valid RGB values ​​of
transparent pixels (see attachment below).

### Tests

A new test image and test case is added:
AcademySoftwareFoundation/OpenImageIO-images#11

<img width="847" height="695" alt="image"
src="https://github.com/user-attachments/assets/484c8b53-5a1b-4732-b95c-55a2f52021b5"
/>


Signed-off-by: Lumina Wang <lumina.wang@autodesk.com>

Author: adskWangl

src/gif.imageio/gifinput.cpp

@@ -366,12 +366,13 @@ GIFInput::read_subimage_data()
                 }
                 int x   = window_left + wx;
                 int idx = m_spec.nchannels * (y * m_spec.width + x);
-                if (0 <= x && x < m_spec.width
-                    && fscanline[wx] != m_transparent_color) {
+                if (0 <= x && x < m_spec.width) {
                     m_canvas[idx]     = colormap[fscanline[wx]].Red;
                     m_canvas[idx + 1] = colormap[fscanline[wx]].Green;
                     m_canvas[idx + 2] = colormap[fscanline[wx]].Blue;
-                    m_canvas[idx + 3] = 0xff;
+                    m_canvas[idx + 3] = (fscanline[wx] != m_transparent_color)
+                                            ? 0xff
+                                            : 0x00;
                 }
             }
         }

testsuite/gif/ref/out.txt

@@ -90,6 +90,12 @@ Reading ../oiio-images/gif/gif_test_loop_count.gif
     gif:LoopCount: 12345
     oiio:ColorSpace: "srgb_rec709_scene"
     oiio:LoopCount: 12345
+Reading ../oiio-images/gif/gif_transparent_rgb.gif
+../oiio-images/gif/gif_transparent_rgb.gif :    2 x    1, 4 channel, uint8 gif
+    SHA-1: 0F690404DA2B75EFC8CE225B19C0F49FEF39F9DE
+    channel list: R, G, B, A
+    gif:Interlacing: 0
+    oiio:ColorSpace: "srgb_rec709_scene"
 Reading tahoe-tiny.gif
 tahoe-tiny.gif       :  128 x   96, 4 channel, uint8 gif
     SHA-1: B05CB30B2108904DAAD7B39E1BD7AA68D1D844CC

testsuite/gif/run.py

@@ -8,7 +8,7 @@
          "gif_tahoe.gif", "gif_tahoe_interlaced.gif",
          "gif_bluedot.gif", "gif_diagonal_interlaced.gif",
          "gif_triangle_interlaced.gif", "gif_test_disposal_method.gif",
-         "gif_test_loop_count.gif"]
+         "gif_test_loop_count.gif", "gif_transparent_rgb.gif"]
 for f in files:
     command += info_command (OIIO_TESTSUITE_IMAGEDIR + "/" + f)