ShaderTools: intro docs for AbstractConverter.

Now that we have AnyShaderConverter we can finally do that.

doc/snippets/MagnumShaderTools.cpp

@@ -37,6 +37,64 @@
 using namespace Magnum;
 
 int main() {
+{
+/* [AbstractConverter-usage-validation] */
+PluginManager::Manager<ShaderTools::AbstractConverter> manager;
+Containers::Pointer<ShaderTools::AbstractConverter> converter =
+    manager.loadAndInstantiate("AnyShaderConverter");
+
+bool valid;
+Containers::String message;
+if(converter) std::tie(valid, message) =
+    converter->validateFile(ShaderTools::Stage::Unspecified, "file.spv");
+if(!converter || !valid)
+    Error{} << "Validation failed:" << message;
+else if(!message.isEmpty())
+    Warning{} << "Validation succeeded with warnings:" << message;
+else
+    Debug{} << "Validation passed";
+/* [AbstractConverter-usage-validation] */
+}
+
+{
+PluginManager::Manager<ShaderTools::AbstractConverter> manager;
+/* [AbstractConverter-usage-compilation] */
+Containers::Pointer<ShaderTools::AbstractConverter> converter =
+    manager.loadAndInstantiate("GlslToSpirvShaderConverter");
+
+/* Using CORRADE_LINE_STRING will make the compiler report line info that
+   matches the source */
+Containers::StringView glsl = "#line " CORRADE_LINE_STRING "\n" R"GLSL(
+#version 450 core
+
+layout(binding=0) uniform Material {
+    vec4 color;
+};
+
+#ifdef TEXTURED
+layout(binding=1) uniform sampler2D colorTexture;
+layout(location=0) in vec2 textureCoordinates;
+#endif
+
+layout(location=0) out vec4 fragmentColor;
+
+void main() {
+    fragmentColor = color
+        #ifdef TEXTURED
+        *texture(colorTexture, textureCoordinates)
+        #endif
+        ;
+}
+)GLSL";
+
+converter->setDefinitions({
+    {"TEXTURED", ""}
+});
+Containers::Array<char> spirv = converter->convertDataToData(
+    ShaderTools::Stage::Fragment, glsl);
+/* [AbstractConverter-usage-compilation] */
+}
+
 {
 Containers::Pointer<ShaderTools::AbstractConverter> converter;
 Containers::Array<const char> extract(const std::string&, const std::string&);

src/Magnum/ShaderTools/AbstractConverter.h

@@ -322,10 +322,49 @@ Shader converters are most commonly implemented as plugins. Depending on
 exposed @ref features(), a plugin can support shader validation, conversion or
 linking.
 
+@m_class{m-block m-warning}
+
+@par Multiple shader sources
+    Compared to the (very broad) OpenGL API, only one shader source can be
+    specified for a single stage. If you need to pass additional preprocessor
+    flags, it's possible to do it via @ref setDefinitions() for plugins that
+    support @ref ConverterFeature::Preprocess; if you have shader source
+    scattered across multiple files either concatenate them together before
+    processing or @cpp #include @ce the dependencies from the top-level file,
+    potentially together with setting up @ref ShaderTools-AbstractConverter-usage-callbacks "file callbacks"
+    (providing the particular converter implementation supports preprocessor
+    includes).
+
 @subsection ShaderTools-AbstractConverter-usage-validation Shader validation
 
+As is common with other plugin interfaces, the
+@ref AnyConverter "AnyShaderConverter" will detect a shader format based on
+file extension, load an appropriate validator plugin for given format and
+validate it:
+
+@snippet MagnumShaderTools.cpp AbstractConverter-usage-validation
+
+In most cases, the validation result depends on the format version and target
+environment used. Formats and versions set by @ref setInputFormat() /
+@ref setOutputFormat() get propagated by @ref AnyConverter "AnyShaderConverter"
+to the particular plugin, however the interpretation is plugin-specific, so be
+sure to read their docs.
+
 @subsection ShaderTools-AbstractConverter-usage-conversion Shader conversion and linking
 
+The following example shows converting a GLSL shader to SPIR-V together with
+setting preprocessor defines and treating any warnings as errors. This time
+it's not using @ref AnyConverter "AnyShaderConverter" but instead asking for a
+plugin using the `GlslToSpirvShaderConverter` alias (which maps for example to
+@ref GlslangConverter "GlslangShaderConverter") --- since we're operating
+directly on data, the plugin would have no chance to know the desired input /
+output format otherwise. Same goes for the shader stage, which has to be
+supplied explicitly:
+
+@snippet MagnumShaderTools.cpp AbstractConverter-usage-compilation
+
+@todoc document linking when SpirvToolsShaderConverter implements that
+
 @subsection ShaderTools-AbstractConverter-usage-callbacks Loading shaders from memory, using file callbacks
 
 Besides loading shaders directly from the filesystem using @ref validateFile()