font sizes are limited to u8 (max size = 255 points)

Fixes #1618

Font sizes in configuration were always a u8, but the keybinding and
internal state was a u16 so it allowed for an ever-growing font size. At
a certain point, there is an integer overflow which causes it to wrap
around. This is all silly, 255 should be large enough for anyone[1]

[1]: Ready to be super wrong about this

src/font/face.zig

@@ -31,7 +31,7 @@ pub const Options = struct {
 /// The desired size for loading a font.
 pub const DesiredSize = struct {
     // Desired size in points
-    points: u16,
+    points: u8,
 
     // The DPI of the screen so we can convert points to pixels.
     xdpi: u16 = default_dpi,

src/input/Binding.zig

@@ -158,8 +158,8 @@ pub const Action = union(enum) {
     paste_from_selection: void,
 
     /// Increase/decrease the font size by a certain amount.
-    increase_font_size: u16,
-    decrease_font_size: u16,
+    increase_font_size: u8,
+    decrease_font_size: u8,
 
     /// Reset the font size to the original configured size.
     reset_font_size: void,