Fixes#7893
Previously, custom shader cursor uniforms were only updated when the
cursor glyph was in the front (block) cursor list. This caused non-block
cursors (such as bar, underline, hollow block, and lock) to be missing
from custom shader effects.
This commit adds a helper to the cell contents struct to retrieve the
current cursor glyph from either the front or back cursor lists, and
updates the renderer to use this helper when setting custom shader
uniforms. As a result, custom shaders now receive correct cursor
information for all supported cursor styles.
cc646ecf46