improve string handling performance about imgui-java HOT 3 OPEN

GetStringUTFChars converts Java UTF-16 text to native UTF-8. Although GetStringCritical gives us jchar*. Even that ImGui supports UTF-16: in a form ImWchar type, yet it's not quite what we need for everything, like ImGui#text which requires char*.

We could pass raw bytes[] from Java, yet it still needs conversions to be done. Do you have thoughts about that?

from imgui-java.

Hm... A further complication is Java 9 compact strings, which encodes LATIN1 strings on 8 bits, the rest as UTF16 – neither of which is UTF8 :) I guess ASCII strings could be used directly – it's probably still more efficient to detect if the string is ASCII only (no high bits) than it is to allocate/copy/release a new buffer.

from imgui-java.

Maybe another way to reduce the overhead is to do the copying conversion into a pre-allocated buffer, eg. using GetStringCritical + ImTextStrToUtf8, instead of Get/ReleaseStringUTFChars (which allocates a new one, AFAIK)

from imgui-java.