Refactor string drawing functions

This branch implements general-use functions for drawing strings & measuring string width/height, and replaces the existing string drawing & measuring functions with #defines using these generalized functions, removing a lot of duplicated code in the process.

V_DrawFontStringAtFixed

• x, y: The fixed_t coordinates.
• option: Video flags.
• pscale, vscale: The fixed_t horizontal & vertical scale at which the characters get drawn. These also affect space width and line spacing.
• string: The string to draw.
• font: A font definition (fontdef_t, see below).

V_DrawAlignedFontStringAtFixed

• Similar to the above, but also allows centered and right-aligned drawing, which is set using a text_align enum.
• text_align has 3 options: align_left, align_center and align_right.
• Unlike previous centered & right-aligned string drawing functions, this function properly aligns multi-line strings.

V_FontStringWidth & V_FontStringHeight

• string: The string to measure the width/height of, taking newlines into account.
• option: Video flags.
• font: A font definition (fontdef_t, see below).

Instead of just patch_t arrays, fonts like hu_font or tny_font are now stored as the following structure, with some additional parameters:

fontdef_t

• chars[]: The patch_t array containing all the font's graphics.
• kerning: The distance between each character. Can also be negative, to move characters closer together.
• spacewidth: The width of a space.
• charwidth: The width of one character. Only used for old video flags like V_MONOSPACE and V_OLDSPACING.
• linespacing: The height difference between two lines, used for newlines. (Gets overridden by V_RETURN8, which sets it to 8.)