video/out/gpu/video: fix chroma offset rotation matrix calculation

82231fd mentioned that for the "rot"
transform flip=true must be set, which "makes no sense at all".

The reason this is happening is that the rotation matrix calculation
is only valid for the 2D coordinate system where y axis is 90 degrees
counterclockwise from x axis, but the coordinate system of chroma
offset has its origin at top-left so it is the opposite, which results
in the rotation going to the opposite way. Setting flip=true fixes the
rotation direction, but results in a flipped y coordinate.

Fix this by reversing the rotation angle for chroma offset rotation
matrix calculation. This also allows removing some duplicated code.

Author: na-na-hi

Diff for: video/out/gpu/video.c

@@ -898,7 +898,10 @@ static void pass_get_images(struct gl_video *p, struct video_image *vimg,
 
         if (type == PLANE_CHROMA) {
             struct gl_transform rot;
-            get_transform(0, 0, p->image_params.rotate, true, &rot);
+            // Reverse the rotation direction here because the different
+            // coordinate system of chroma offset results in rotation
+            // in the opposite direction.
+            get_transform(0, 0, 360 - p->image_params.rotate, t->flipped, &rot);
 
             struct gl_transform tr = chroma;
             gl_transform_vec(rot, &tr.t[0], &tr.t[1]);
@@ -908,15 +911,13 @@ static void pass_get_images(struct gl_video *p, struct video_image *vimg,
 
             // Adjust the chroma offset if the real chroma size is fractional
             // due image sizes not aligned to chroma subsampling.
-            struct gl_transform rot2;
-            get_transform(0, 0, p->image_params.rotate, t->flipped, &rot2);
-            if (rot2.m[0][0] < 0)
+            if (rot.m[0][0] < 0)
                 tr.t[0] += dx;
-            if (rot2.m[1][0] < 0)
+            if (rot.m[1][0] < 0)
                 tr.t[0] += dy;
-            if (rot2.m[0][1] < 0)
+            if (rot.m[0][1] < 0)
                 tr.t[1] += dx;
-            if (rot2.m[1][1] < 0)
+            if (rot.m[1][1] < 0)
                 tr.t[1] += dy;
 
             off[n] = tr;