Replace UTF-8 decoder with branchless version by Christopher Wellons.

Decoding performance increase ranges from 30-40%.

Changes:
* Errors handling near the end of string changed. If input does not contain enough bytes, decoder returns `IM_UNICODE_CODEPOINT_INVALID`, consuming all remaining bytes while old decoder consumed only one byte.

Guarantees:
* At least one byte is consumed, if input had at least one byte available.
* Number of consumed bytes will never seek past end of string.

Requirements:
* `in_text` is a valid pointer.
* String pointed by `in_text` must be zero-terminated, or `in_text_end` is not NULL.

docs/CHANGELOG.txt

@@ -59,6 +59,9 @@ Other Changes:
 - Tab Bar: Do not display a tooltip if the name already fits over a given tab. (#3521)
 - Drag and Drop: Fix drag and drop to tie same-size drop targets by choosen the later one. Fixes dragging
   into a full-window-sized dockspace inside a zero-padded window. (#3519, #2717) [@Black-Cat]
+- Misc: Replaced UTF-8 decoder by branchless one by Christopher Wellons (30~40% faster). [@rokups]
+  Super minor fix handling incomplete UTF-8 contents: if input does not contain enough bytes, decoder 
+  returns IM_UNICODE_CODEPOINT_INVALID and consume remaining bytes (vs old decoded consumed only 1 byte).
 - Backends: OpenGL3: Use glGetString(GL_VERSION) query instead of glGetIntegerv(GL_MAJOR_VERSION, ...) 
   when the later returns zero (e.g. Desktop GL 2.x). (#3530) [@xndcn]
 - Backends: OpenGL3: Backup and restore GL_PRIMITIVE_RESTART state. (#3544) [@Xipiryon]