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.

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.

Currently Alacritty only works on hardware which supports OpenGL 3.3 or
more, which can become problematic with older devices. This patch adds a
new GLES2 renderer, since it is much more widely supported, especially
on weaker hardware like phones or a Raspberry Pi.

While the GLES2 renderer is slower than the OpenGL 3.3+ version, it is
still significantly faster than software rendering. However because of
this performance difference it is only used when necessary and there
should be no difference for machines supporting OpenGL 3.3+.

The two renderers are largely independent and separated in the
`renderer/text/glsl3` and `renderer/text/gles2` modules. Separate
shaders are also required for text rendering. The rectangle rendering
for underlines and the visual bell works identically for both versions,
but does have some version-specific shader code.

Fixes #128.

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

This finishes implementation of underline styles provided by
`CSI 4 : [1-5] m` escape sequence.