Allow env OPENIMAGEIO_OPTIONS to set global OIIO::attribute()

Akin to how we have `OPENIMAGEIO_IMAGECACHE_OPTIONS` and
`OPENIMAGEIO_TEXTURE_OPTIONS`.

`OPENIMAGEIO_TEXTURE_OPTIONS` is a comma-separated list of name=val
pairs that upon startup will seed the global options that are usually
set by the OIIO::attribute() call.

Along the way, I beefed up the OptParser to be able to handle both
single and double quotes equally well. That lets you set an option that
is a string, that itself contains quotes.

Also document all of these options -- I realized the PDF docs don't talk
about even the existing env variables.

src/doc/imageioapi.tex

@@ -1115,6 +1115,19 @@ \section{Miscellaneous Utilities}
 
 \noindent The following are the recognized attributes:
 
+\apiitem{string options}
+\vspace{10pt}
+This catch-all is simply a comma-separated list of {\cf name=value}
+settings of named options, which will be parsed and individually set.
+For example,
+\begin{code}
+        OIIO::attribute ("options", "threads=4,log_times=1");
+\end{code}
+Note that if an option takes a string value that must itself contain a
+comma, it is permissible to enclose the value in either single ({\cf ' '})
+or double ({\cf " "}) quotes.
+\apiend
+
 \apiitem{int threads}
 \vspace{10pt}
 \index{threads} \label{sec:attribute:threads}
@@ -1349,6 +1362,21 @@ \section{Environment variables}
 \product at times that it is not convenient to set options individually from
 inside the source code.
 
+\apiitem{OPENIMAGEIO_OPTIONS}
+\NEW % 2.0
+Allows you to seed the global \product-wide options.
+
+The value of the environment variable should be a comma-separated list of
+\emph{name=value} settings. If a value is a string that itself needs to
+contain commas, it may be enclosed in single or double quotes.
+
+Upon startup, the contents of this environment variable will be passed
+to a call to:
+\begin{code}
+    OIIO::attribute ("options", value);
+\end{code}
+\apiend
+
 \apiitem{OPENIMAGEIO_IMAGECACHE_OPTIONS}
 Allows you to seed the options for any \ImageCache created.
 

src/include/OpenImageIO/optparser.h

@@ -74,11 +74,11 @@ optparse1(C& system, const std::string& opt)
     // otherwise treat it as a string
 
     // trim surrounding double quotes
-    if (value.size() >= 2 && value[0] == '\"'
-        && value[value.size() - 1] == '\"')
+    if (value.size() >= 2 && (value[0] == '\"' || value[0] == '\'')
+        && value[value.size() - 1] == value[0])
         value = std::string(value, 1, value.size() - 2);
 
-    return system.attribute(name.c_str(), value.c_str());
+    return system.attribute(name, value);
 }
 
 
@@ -98,12 +98,17 @@ optparser(C& system, const std::string& optstring)
     size_t pos = 0;
     while (pos < len) {
         std::string opt;
-        bool inquote = false;
+        char inquote = 0;
         while (pos < len) {
             unsigned char c = optstring[pos];
-            if (c == '\"') {
-                // Hit a double quote -- toggle "inquote" and add the quote
-                inquote = !inquote;
+            if (c == inquote) {
+                // Ending a quote
+                inquote = 0;
+                opt += c;
+                ++pos;
+            } else if (c == '\"' || c == '\'') {
+                // Found a quote
+                inquote = c;
                 opt += c;
                 ++pos;
             } else if (c == ',' && !inquote) {

src/libOpenImageIO/imageio.cpp

@@ -40,6 +40,7 @@
 #include <OpenImageIO/fmath.h>
 #include <OpenImageIO/hash.h>
 #include <OpenImageIO/imageio.h>
+#include <OpenImageIO/optparser.h>
 #include <OpenImageIO/parallel.h>
 #include <OpenImageIO/strutil.h>
 #include <OpenImageIO/sysutil.h>
@@ -143,6 +144,27 @@ class TimingLog {
 };
 static TimingLog timing_log;
 
+
+
+// Pipe-fitting class to set global options, for the sake of optparser.
+struct GlobalOptSetter {
+public:
+    template<typename T> bool attribute(string_view name, T val) const
+    {
+        return OIIO::attribute(name, val);
+    }
+};
+
+
+// Trick to force get the "OPENIMAGEIO_OPTIONS" env var on startup.
+bool force_global_opts = []() {
+    auto options = Sysutil::getenv("OPENIMAGEIO_OPTIONS");
+    // std::cout << "OPTIONS: '" << options << "'\n";
+    if (!options.empty())
+        attribute("options", options);
+    return true;
+}();
+
 }  // namespace
 
 
@@ -283,6 +305,10 @@ pvt::log_time(string_view key, const Timer& timer)
 bool
 attribute(string_view name, TypeDesc type, const void* val)
 {
+    if (name == "options" && type == TypeDesc::STRING) {
+        GlobalOptSetter gos;
+        return optparser(gos, *(const char**)val);
+    }
     if (name == "threads" && type == TypeInt) {
         int ot = Imath::clamp(*(const int*)val, 0, maxthreads);
         if (ot == 0)