tools/player: fix initial alpha on the progress bar
tools/player: fix initial alpha on the progress bar

At startup, if the cursor is not hover the window, the text box
background is visible (not the text because its content has not been
written). This commit makes sure the initial state is with 0 alpha such
that the bar doesn't appear until the first over.

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl