WIP: Fix GLSL renderer.
Tweak alphas to match GLWaveformRendererFilteredSignal.
Make framebuffers 4x the size of the widget and downsample with linear interpolation.
Add logic to WaveformRendererSignalBase to get per-band gains.
Make right-clicks on the GLSL widget call debugClick().
Delete max-signal shader because we don't need 2-passes anymore.
No need for visual indices when rendering the final quad.
Pop matrices after we push them.
Draw the horizontal axis in GLSL waveforms.
Remove some old comments and use the common gain calculation method.
Use WaveformRendererSignalBase::getGains to calculate signal gains in all renderers.