Tweak gamepad viewer example style

[rparrett]

Objective

Since #10339, the contrast in this example has been very low. While I was in there, I made a few other tweaks to the style.

Alternative to #10102.

Solution

• Increase brightness of inactive buttons for higher contrast on the new clear color
• Combine DEAD_COLOR and EXTENT_COLOR. These were using the same value, and combining them might make the intent a little clearer. (This is the single color for "not the live zone.")
• Make the "stick buttons" slightly smaller, so it's a bit more obvious that they are sitting inside of the default dead zone.
• Remove explicit text color -- it was the same as the default
• Add top-left margin to the text in the top left, and change the font size to better match other examples with text in the corner.

Screenshots

With Bevy's default dead / live zones.

Before / After

With Bevy's old dead / live zones. (with the upper live zone boundary != 1.0)

Before / After