[sensor_msgs] Fix row_step when resizing organized PointCloud2

[1visio]

Fixes #320

PointCloud2Modifier::resize(width, height) reused the flat resize(size)
path, then updated width and height without recomputing row_step. This
left row_step equal to the total data size instead of one row's byte size.

Set row_step to width * point_step and add a regression test covering a
non-square organized cloud and its backing data size.

Validation:

• git diff --check
• ROS 2 tests not run locally because colcon and ros2 are unavailable.

[mergify]

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[ahcorde]

Pulls: #340
Gist: https://gist.githubusercontent.com/ahcorde/a5cabfb9f07efe8be1faf9469c5e0a2f/raw/ff1673765ba43f6862499146d80f66bfb4a008c1/ros2.repos
BUILD args: --packages-above-and-dependencies sensors_msgs
TEST args: --packages-above sensors_msgs
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/20231

• Linux
• Linux-aarch64
• Linux-rhel
• Windows

[ahcorde]

Pulls: #340
Gist: https://gist.githubusercontent.com/ahcorde/86a3805f2a47f73768f55576fad73485/raw/ff1673765ba43f6862499146d80f66bfb4a008c1/ros2.repos
BUILD args: --packages-above-and-dependencies sensor_msgs
TEST args: --packages-above sensor_msgs
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/20236

• Linux
• Linux-aarch64
• Linux-rhel
• Windows