Auto merge of #6373 - robertknight:gh5660-rob-x11_scroll_speed, r=pcwalton

Use a faster scroll speed under X11

Platforms may report scroll deltas either in
chunks/lines/rows or pixels, depending on the
platform API and device capabilities.

If the platform reports a line/chunk-based delta
then the application needs to convert the delta
into a suitable number of pixels. Apple's documentation for example states
that the app should interpret the delta as a number of lines or rows to scroll,
depending on the type of view.

This commit just hardcodes it to 57 as
a starting point which matches the value that
Firefox calculates as the max char height
for the root frame on my system.

This depends on this Glutin PR: https://github.com/tomaka/glutin/pull/483

Fixes #5660

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