content: Adjust jetpack exhaust emission.

Part of fixing <#504>.
Previously, emission would be reduced by alpha, which I am now defining
to be the wrong approach. So, within the new paradigm, the jetpack
exhaust needs to be dimmer to produce the desired visual results.
The good(?) news is, this means the jetpack no longer lights up brightly
because the new rendering better matches what light propagation does.

This commit makes the renderer tests pass again by blessing the new
`icons` output.

all-is-cubes/src/inv/icons.rs

@@ -209,7 +209,7 @@ impl Icons {
                     let exhaust = if active {
                         Block::builder()
                             .color(rgba_const!(1.0, 1.0, 1.0, 0.1))
-                            .light_emission(rgb_const!(1.0, 0.8, 0.8) * 10.0)
+                            .light_emission(rgb_const!(1.0, 0.8, 0.8) * 1.0)
                             .build()
                     } else {
                         AIR