This removes the if and lowers amount of operations.

This improves undercurl rendering preserving its original thickness.
This also makes it look not out-of place when places next to other
lines.

Some old hardware doesn't like universal shader approach for all the
rectangle kinds leading to ALU instruction limits. This commit fixes
it by splitting the shader per rectangle kind.

Fixes #6417.

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.

GLES2 has GL_EXT_blend_func_extended extension that enables
dual-source blending, so essentially we can reuse fragment shader
from GLSL3 renderer and do 1 rendering pass instead of 3 for the
text.

Co-authored-by: Kirill Chibisov <contact@kchibisov.com>
Co-authored-by: Christian Duerr <contact@christianduerr.com>

Since `x` position in rect shader represents left side of the pixel we
should use the center of it when dealing with contiguous functions.

Ceiling line position results in strikeout line being lower than
it should.