Fixes alacritty#910

We still add a reference to the graphic in the first cell of every
line under the image, but we don’t erase any of the text in any of the
cells.

after printing a sixel image. The default is for the cursor to be
moved to the first column of the line after the image. When we receive
CSI ? 8452 h, we will instead leave the cursor on the last line of the
image, on the next column past the end of the image.

…mber

From #6

This is reverse of the *sixel scrolling* option, which should match the actual
behaviour of DEC terminals.

For reference: alacritty#4763 (comment)

The merge requires multiple changes in order to be compatible with the last
version of Alacritty:

- Textures are now always deleted on Drop.

  This is required because Alacritty now supports multiple windows in the same
  process, so we can't assume that all resources are freed when a single window
  is closed. This is the same approach used for the atlas textures.

- The graphics feature is only compatible with OpenGL 3.3.

  Alacritty now supports GLES 2.2, but in order to provide a proper support for
  it we need a different approach, specific for that version.

- Cell dimensions in pixels are re-added to the alacritty_terminal crate.

With the template we can create hyperlinks attached to the graphic.

To avoid reflow issues when a row is shrank, wrapped rows that only contain
graphic cells are discarded. With this approach we loss some info, like the
hyperlink, but the image is always properly positioned in the grid.

When text is added to a cell with a reference to a graphic, an operation is sent
to the OpenGL thread to replace a subregion of the cell with a transparent area.

If the OpenGL driver supports the GL_ARB_clear_texture extension, the region is
updated with glClearTexSubImage. If the extension is not available, the
texture is updated with glTexSubImage2D.

Similar to the underline line rendered when the cursor is over an hyperlink, for
graphics we now render a border around the graphic.

If a graphic is added over another one, the implementation now checks if new
graphic has transparent pixels in every cell. If so, the graphic is appended to
the cell, instead of replacing the previous one.

SmallVec is used to prevent heap allocation when the cell only contains a single
graphic. This should be the most common scenario.

The GPU will store up to 100 textures. If another texture is added when there
are 100, the oldest one is deleted.

A simple optimization for inserting graphics is to detect when a new graphic is
replacing completely an existing one. If both graphics have the same size, and
the new one is opaque, we can assume that the previous graphic will not be
displayed anymore, so it is not considered when update the graphics list in a
single cell.

This commit also adds serde implementation for GraphicsCell. This was used to
debug the new logic.

- Reimplement abs_diff().
- Use positional arguments to format the error message in assert_color!().

This fixes a bug that crashes the terminal when a graphic is added before
resizing the window.

Support GLES2 Renderer in sixel

The limit per grid is increased to `1000`, and a new limit per cell is added,
set to `20`.

Suggested by clippy.

… area

CSI XTSMGRAPHICS Pi=2, Pa=1 should return dimensions that fit in text area

The response for `\e[c` (Send Device Attributes) now returns level 2 with the
Sixel extension.

The other extensions are 6 (Selectively Erasable Characters) and 22 (Color Text).
The values are documented in page 04-19 of DEC-STD-070.

With `$ cargo update vte`.