[flatland] Fix Pixel Scale Calculation

Pixel Scale calculation in the LinkSystem was incorrectly
extracting the scale value from matrices, and including
translations in the calculation, causing unnecessarily large
scales to be returned to clients.

Fixed: 90588

Change-Id: I5322bd99437e66013a43a7ae14421f6d55737c6c
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/680137
Commit-Queue: Chris Fontas <cfontas@google.com>
Reviewed-by: Emircan Uysaler <emircan@google.com>

Author: cfontas

src/ui/scenic/lib/flatland/link_system.cc

@@ -20,10 +20,20 @@ namespace flatland {
 
 namespace {
 
+// Scale can be extracted from a matrix by finding the length of the
+// column the scale is located in:
+//
+//   a b c
+//   e f g
+//   i j k
+//
+// If |a| is the x scale and rotation, and |f| is the y scale and rotation, then
+// we can calculate the x scale with length(vector(a,e,i)) and y scale with
+// length(vector(b,f,j)).
 glm::vec2 ComputeScale(const glm::mat3& matrix) {
-  const glm::vec3 x_row = glm::row(matrix, 0);
-  const glm::vec3 y_row = glm::row(matrix, 1);
-  return {std::fabs(glm::length(x_row)), std::fabs(glm::length(y_row))};
+  const glm::vec3 x_column = glm::column(matrix, 0);
+  const glm::vec3 y_column = glm::column(matrix, 1);
+  return {glm::length(x_column), glm::length(y_column)};
 }
 
 }  // namespace