GL: unconditionally trim shader compilation/linking messages.

Until now it relied on the driver adding a \n after each message. Which
happened in 99% cases, but sometimes not, and it is really annoying in
that case. Right now I witnessed Mesa giving me a non-newlined message
if I call checkLink() and a shader compilation failed before, resulting
in nasty output like

    error: linking with uncompiled/unspecialized shaderAssertion checkLink() failed at /home/mosra/Code/magnum/src/Magnum/Shaders/FlatGL.cpp:248

And I bet I encountered other weird cases, like there being two
newlines, or the message being just a newline without any other content,
thus spamming the output on every compilation. It's not practical to try
to find driver-specific workarounds here, so I'm just trimming always
and then rely on Debug adding its own newline after.

src/Magnum/GL/AbstractShaderProgram.cpp

@@ -31,6 +31,7 @@
 #ifndef MAGNUM_TARGET_WEBGL
 #include <Corrade/Containers/String.h>
 #endif
+#include <Corrade/Containers/StringStl.h> /** @todo remove once <string>-free */
 #include <Corrade/Containers/Reference.h>
 #include <Corrade/Utility/DebugStl.h>
 
@@ -606,17 +607,24 @@ bool AbstractShaderProgram::checkLink() {
        be said, handle that as well. */
     Context::current().state().shaderProgram.cleanLogImplementation(message);
 
+    /* Usually the driver messages contain a newline at the end. But sometimes
+       not, such as in case of a program link error due to shaders not being
+       compiled yet on Mesa; sometimes there's two newlines, sometimes just a
+       newline and nothing else etc. Because trying do this in driver-specific
+       workarounds would involve an impossible task of checking all possible
+       error messages on every possible driver, just trim all whitespace around
+       the message always and let Debug add its own newline. */
+    const Containers::StringView messageTrimmed = Containers::StringView{message}.trimmed();
+
     /* Show error log */
     if(!success) {
-        Error out{Debug::Flag::NoNewlineAtTheEnd};
-        out << "GL::AbstractShaderProgram::link(): linking failed with the following message:"
-            << Debug::newline << message;
+        Error{} << "GL::AbstractShaderProgram::link(): linking failed with the following message:"
+            << Debug::newline << messageTrimmed;
 
     /* Or just warnings, if any */
-    } else if(!message.empty()) {
-        Warning out{Debug::Flag::NoNewlineAtTheEnd};
-        out << "GL::AbstractShaderProgram::link(): linking succeeded with the following message:"
-            << Debug::newline << message;
+    } else if(messageTrimmed) {
+        Warning{} << "GL::AbstractShaderProgram::link(): linking succeeded with the following message:"
+            << Debug::newline << messageTrimmed;
     }
 
     return success;

src/Magnum/GL/Shader.cpp

@@ -783,17 +783,26 @@ bool Shader::checkCompile() {
        be said, handle that as well. */
     Context::current().state().shader.cleanLogImplementation(message);
 
+    /* Usually the driver messages contain a newline at the end. But sometimes
+       not, such as in case of a program link error due to shaders not being
+       compiled yet on Mesa; sometimes there's two newlines, sometimes just a
+       newline and nothing else etc. Because trying do this in driver-specific
+       workarounds would involve an impossible task of checking all possible
+       error messages on every possible driver, just trim all whitespace around
+       the message always and let Debug add its own newline. */
+    const Containers::StringView messageTrimmed = Containers::StringView{message}.trimmed();
+
     /* Show error log */
     if(!success) {
-        Error out{Debug::Flag::NoNewlineAtTheEnd};
-        out << "GL::Shader::compile(): compilation of" << shaderName(_type) << "shader"
-            << "failed with the following message:" << Debug::newline << message;
+        Error{} << "GL::Shader::compile(): compilation of" << shaderName(_type)
+            << "shader failed with the following message:" << Debug::newline
+            << messageTrimmed;
 
     /* Or just warnings, if any */
-    } else if(!message.empty()) {
-        Warning out{Debug::Flag::NoNewlineAtTheEnd};
-        out << "GL::Shader::compile(): compilation of" << shaderName(_type) << "shader"
-            << "succeeded with the following message:" << Debug::newline << message;
+    } else if(messageTrimmed) {
+        Warning{} << "GL::Shader::compile(): compilation of" << shaderName(_type)
+            << "shader succeeded with the following message:" << Debug::newline
+            << messageTrimmed;
     }
 
     return success;