Avoid flattening matrix row storage

Address matrix coefficients through their declared row and column bounds. This preserves generated code while avoiding pointer arithmetic beyond the first row subobject in both 3x3 and 3x4 transform stages.

Bug: https://github.com/libjxl/libjxl/issues/4949

Assisted-by: OpenAI Codex
Change-Id: Ied7f5ff71ecb7ea3cedb5e790b5c377b26f42685
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/1333076
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>

src/Transform_inl.h

diff --git a/src/Transform_inl.h b/src/Transform_inl.h
index eb5c5ff..53a35ef 100644
--- a/src/Transform_inl.h
+++ b/src/Transform_inl.h
@@ -1113,11 +1113,9 @@
 }
 
 STAGE(matrix_3x3, const skcms_Matrix3x3* matrix) {
-    const float* m = &matrix->vals[0][0];
-
-    F R = m[0]*r + m[1]*g + m[2]*b,
-      G = m[3]*r + m[4]*g + m[5]*b,
-      B = m[6]*r + m[7]*g + m[8]*b;
+    F R = matrix->vals[0][0]*r + matrix->vals[0][1]*g + matrix->vals[0][2]*b,
+      G = matrix->vals[1][0]*r + matrix->vals[1][1]*g + matrix->vals[1][2]*b,
+      B = matrix->vals[2][0]*r + matrix->vals[2][1]*g + matrix->vals[2][2]*b;
 
     r = R;
     g = G;
@@ -1125,11 +1123,12 @@
 }
 
 STAGE(matrix_3x4, const skcms_Matrix3x4* matrix) {
-    const float* m = &matrix->vals[0][0];
-
-    F R = m[0]*r + m[1]*g + m[ 2]*b + m[ 3],
-      G = m[4]*r + m[5]*g + m[ 6]*b + m[ 7],
-      B = m[8]*r + m[9]*g + m[10]*b + m[11];
+    F R = matrix->vals[0][0]*r + matrix->vals[0][1]*g + matrix->vals[0][2]*b
+        + matrix->vals[0][3],
+      G = matrix->vals[1][0]*r + matrix->vals[1][1]*g + matrix->vals[1][2]*b
+        + matrix->vals[1][3],
+      B = matrix->vals[2][0]*r + matrix->vals[2][1]*g + matrix->vals[2][2]*b
+        + matrix->vals[2][3];
 
     r = R;
     g = G;