Rename gfx/render_task.rs -> gfx/paint_task.rs
Rename RenderTask -> PaintTask.
Rename canvas_render_task -> canvas_paint_task
Rename CanvasRenderTask -> CanvasPaintTask.
Rename RenderContext -> PaintContext
Rename RenderLayer -> PaintLayer
Rename RenderRequest -> PaintRequest
Rename RenderChan -> PaintChan
Rename gfx::paint_task::Msg
Rename compositor_task::Msg.
Rename RenderListener -> PaintListener, and its methods.
Rename RenderState -> PaintState
Rename IOCompositor's members.
Rename WindowMethods::set_paint_state()
Fix README.md: a tile size is used for 'painting'.
Rename RendererReadyMsg -> PainterReadyMsg
Rename RenderFontCacheEntry -> PaintFontCacheEntry.
Rename TaskState::RENDER -> TaskState::PAINT
Replace almost "render" to "paint" words in gfx crate.
Replace almost "render" to "paint" in compositing crate.

This doesn't touch some "render" words which are used as general means.