Update coordinate frame naming

[cmbruns]

It's time for some better three-letter shorthand names for coordinate frames used internally by vimage.

old name	new name	units	origin	Y dir	forward -Z	description
omp	opx	pixels	top left	down	N/A	logical image pixels considering EXIF orientation
obq	usr	radians	user eye	up	app center pixel	app user's 3D view frame
ont	geo	radians	photographer's camera	sky zenith	north	geographic orientation from GPano:PoseHeading etc.
raw	pcm	radians	photographer's camera	camera top	lens forward	photographer's camera
N/A	ivw	radians	photographer's camera	artistic up	artistic forward	initial panorama view in app from GPano:InitialViewHeading etc.
tco	otc	images	top left	down	N/A	logical full image texture coordinates accounting for EXIF orientation
tcr	rtc	images	top left	down	N/A	internal full image texture coordinates ready for glsl texture(...)
tct	ttc	tiles	top left	down	N/A	tile texture coordinate ready for glsl texture(...)

[cmbruns]

The current set of frame names is maintained in https://github.com/cmbruns/vimage/blob/main/doc/vimage_transforms.ipynb