Reduce reallocation texture of subtitle font

Subtitle character cache texture is initialized for 100 letters.
Actual size is determined by subtitle font size.
Texture will be overwritten if cache is full.

If text length at the same is exceeded 100 letters, texture size will be increased to text length.
But this will not happen normally.

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Subtitle text have two fonts. (m_font, m_borderFont).
Each font have a texture for font atlas.

When character caches are full, they will reallocate their textures.
Video memory is fragmented by two growing textures.

Defragment is not fast enough to avoid stuttering on some system.
Note that CHARS_PER_TEXTURE_LINE is a guess for the size needed - I think it's set at 64 by default?  For larger subtitles with a small-ish MaxTextureSize() this may be smaller than 64.  So a more accurate measure of the number of characters per row can be found based on texture width and using GetTextureLineHeight() as a proxy (assumes square chars, which is probably reasonably good, slightly conservative (i.e. higher) estimate of average width).