gl: Fix padding in icon library

The previous code would not pad the left side with the left column of
pixels but with the top row.

Test attached.

gsk/gl/gskgliconlibrary.c

@@ -148,6 +148,7 @@ gsk_gl_icon_library_add (GskGLIconLibrary     *self,
                    gl_format, gl_type,
                    pixel_data);
   /* Padding left */
+  glPixelStorei (GL_UNPACK_ROW_LENGTH, width);
   glTexSubImage2D (GL_TEXTURE_2D, 0,
                    packed_x, packed_y + 1,
                    1, height,
@@ -161,7 +162,6 @@ gsk_gl_icon_library_add (GskGLIconLibrary     *self,
                    pixel_data);
 
   /* Padding right */
-  glPixelStorei (GL_UNPACK_ROW_LENGTH, width);
   glPixelStorei (GL_UNPACK_SKIP_PIXELS, width - 1);
   glTexSubImage2D (GL_TEXTURE_2D, 0,
                    packed_x + width + 1, packed_y + 1,

testsuite/gsk/compare/blurred-lines.node

@@ -0,0 +1,6 @@
+texture {
+  bounds: 0 0 20 20;
+  texture: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAoAAAAUCAYAAAC07qxWAAAAHElEQVQokWP8z8D6n4EIwESMolGF\
+1FM4CgYrAACekgIXqju67AAAAABJRU5ErkJggg==\
+");
+}

testsuite/gsk/meson.build

@@ -19,6 +19,7 @@ compare_render_tests = [
   'blend-invisible-child',
   'blend-in-rotate',
   'blend-normal',
+  'blurred-lines',
   'border-bottom-right',
   'border-one-rounded',
   'borders-rotated',