Fix memory access in case of 3-byte pixel formats

[rhaschke]

This fixes an invalid memory access found in #1508 (comment):

While the memory array allocated for Ogre::PixelBox is considering the pixel format size here:

rviz/src/rviz/selection/selection_manager.cpp

Lines 778 to 779 in 171eb3c

	delete[](uint8_t*) dst_box.data;
	dst_box = Ogre::PixelBox(render_w, render_h, 1, format, data);

the unpacking routine always assumes that the pixel format is 4 bytes here:

rviz/src/rviz/selection/selection_manager.cpp

Lines 590 to 593 in 171eb3c

	uint32_t pos = (x + y * w) * 4;
	uint32_t pix_val = *(uint32_t*)((uint8_t*)box.data + pos);
	uint32_t handle = colorToHandle(box.format, pix_val);

This is failing if the pixel format is 3 bytes only...
Unfortunately. PixelBox::getColourAt() transforms the bytes into float values, which we transform back subsequently to bytes... That's a lot more overhead 😢

[wjwwood]

Seems reasonable to me. I don't know enough about this part of the code or this part of Ogre's API to know if there's a more efficient way to do it. It seems like it is just necessary to do this, regardless of the performance issues.