Video Creator: replace "target same pixel ratio"

[jared-hughes]

1. Make "target same pixel ratio" enabled by default. That is what is expected in most cases
2. The alternative should be changed so that the "width" and "height" in the inputs are precisely the width and height of the output image.
3. Rename this to be the checked state, and rename it to "Preserve line width, point radius, text size as pixel distances instead of (whatever is the alternative)" (wordsmithing needed)

Supersedes #663?

Some more discussion

This is something confusing I want to change about DesModder. The confusing thing is there's three numbers to change: the width and height of the output image, as well as how wide Desmos thinks the image is when exporting (affects the widths of lines, radius of points, and size of text). Presumably a better UI would be to control these numbers directly, with the third number defaulting to the width of the output image.

The Desmos screenshot API takes input a width, height, and targetPixelRatio (designed to be either 1 or 2 (for Retina)). The size of the output image is width*targetPixelRatio by height*targetPixelRatio but Desmos thinks it's width width for the widths of lines.

The current behavior with DesModder, if "target same pixel ratio" is checked, is the expected one: the size of the output image is width by height, and Desmos thinks it's width width for the widths of lines.

If "target same pixel ratio" is unchecked (the unfortunate default): DesModder scales numbers so Desmos thinks it's the same width as the real graphpaper (for scaling lines; this is good if the graph relies on the width of lines, etc), and the actual output resolution is scaled proportionally.