Fix loading plain font.png Unicode indexing out of bounds

This code was introduced by Dav999 in
abf12632bb (PR #1077), but it contains a
memory error. I spotted this with Valgrind.

The problem comes from the fact that `max_codepoint` is calculated from
the width and height of the surface (which will have the same width and
height as the source `font.png` from the filesystem). Let's work through
an example using a typical 128 by 128 `font.png` and an 8 by 8 glyph.

`chars_per_line` is calculated by dividing the width of the image
(`temp_surface->w`, or 128) by `f->glyph_w` (8), yielding 16.
`max_codepoint` is calculated by first calculating the height of the
image divided by the height of the glyph - which here just happens to be
the same as `chars_per_line` (16) since we have a square `font.png` -
and then multiplying the result by `chars_per_line`. 16 times 16 is 256.

Now it is important to recognize here that this is the _amount_ of
glyphs in `font.png`. It is _not_ the last codepoint in the image. To
see why, consider the fact that codepoint 0 is contained in the image.
If we have codepoint 0, then we can't have codepoint 256, because that
would imply that we have 257 codepoints, but clearly, we don't. If we
try to read codepoint 256, then after working through the calculations
to read the glyphs, we would be trying to read from pixel columns 0
through 7 and pixel rows 128 through 135... in a 128 by 128 image...
which is clearly incorrect.

Therefore, it's incorrect to write the upper bound of the for-loop
iterating over every codepoint as `codepoint <= max_codepoint` instead
of `codepoint < max_codepoint`.

desktop_version/src/Font.cpp

@@ -407,7 +407,7 @@ static uint8_t load_font(FontContainer* container, const char* name)
             const uint32_t chars_per_line = temp_surface->w / f->glyph_w;
             const uint32_t max_codepoint = (temp_surface->h / f->glyph_h) * chars_per_line;
 
-            for (uint32_t codepoint = 0x00; codepoint <= max_codepoint; codepoint++)
+            for (uint32_t codepoint = 0x00; codepoint < max_codepoint; codepoint++)
             {
                 if (codepoint > 0x7F)
                 {