Linear gradient painting is implemented in the following way:
1. The rectangle is divided into segments where each segment represents
a simple linear gradient between an adjacent pair of stops.
2. Each quad is filled separately using a fragment shader that
interpolates between two colors.
For now `angle` and `repeat_length` parameters are ignored.
Since we already call `Painter::flush()` in `PageHost::paint()` we do
not need to do that again in `PaintingCommandExecutorGPU` destructor.
This makes GPU painting run noticeably faster because `flush()` does
expensive `glReadPixels()` call.
Representing a text run panting command as a vector of glyphs, rather
than as a string simplifies collecting of unique glyphs which is a
prerequisite for `prepare_glyphs_texture()` call.
In the upcoming changes, Painter will be used to store the state of
OpenGL context. For example, if Painter is aware of the shader that
have already been loaded, it will be possible to reuse them across
repaints. Also, it would be possible to manage state of loaded textures
and add/remove them depending on which ones are present in the next
sequence of painting commands.
