GL: expose Renderer::Feature::SampleAlphaToCoverage and related APIs.

I need alpha to coverage for rendering line caps with MSAA, took it as
an opportunity to expose the other related features as well. Fun how I
survived 12+ years without needing those.

doc/changelog.dox

@@ -149,6 +149,10 @@ See also:
 -   Exposed @gl_extension{ARB,buffer_storage} as
     @ref GL::Buffer::setStorage() together with additions to
     @ref GL::Buffer::MapFlag
+-   Exposed missing @ref GL::Renderer::Feature::SampleAlphaToCoverage,
+    @relativeref{GL::Renderer,Feature::SampleAlphaToOne},
+    @relativeref{GL::Renderer,Feature::SampleCoverage} and
+    @ref GL::Renderer::setSampleCoverage() GL 1.3 APIs
 -   A new @cpp "nv-egl-crashy-query-device-attrib" @ce workaround for a crash
     happening during EGL initialization in recent NVidia drivers.  See
     @ref opengl-workarounds and [mosra/magnum#491](https://github.com/mosra/magnum/pull/491)

doc/opengl-mapping.dox

@@ -352,7 +352,7 @@ OpenGL function                         | Matching API
 
 OpenGL function                         | Matching API
 --------------------------------------- | ------------
-@fn_gl{SampleCoverage}                  | |
+@fn_gl{SampleCoverage}                  | @ref GL::Renderer::setSampleCoverage()
 @fn_gl{SampleMaski}                     | |
 @fn_gl{SamplerParameter}                | |
 @fn_gl{Scissor}                         | @ref GL::Renderer::setScissor()

src/Magnum/GL/Renderer.cpp

@@ -147,6 +147,10 @@ void Renderer::setPointSize(const Float size) {
 }
 #endif
 
+void Renderer::setSampleCoverage(const Float value, const bool invert) {
+    glSampleCoverage(value, invert);
+}
+
 #if !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
 void Renderer::setMinSampleShading(const Float value) {
     Context::current().state().renderer.minSampleShadingImplementation(value);

src/Magnum/GL/Renderer.h

@@ -450,6 +450,38 @@ class MAGNUM_GL_EXPORT Renderer {
             RasterizerDiscard = GL_RASTERIZER_DISCARD,
             #endif
 
+            /**
+             * Transforms a fragment's alpha value into a coverage value to
+             * determine its contribution to the final pixel value in a
+             * multisampled framebuffer.
+             * @m_since_latest
+             * @see @ref Feature::SampleAlphaToOne,
+             *      @ref Feature::SampleCoverage, @ref setSampleCoverage()
+             */
+            SampleAlphaToCoverage = GL_SAMPLE_ALPHA_TO_COVERAGE,
+
+            #ifndef MAGNUM_TARGET_GLES
+            /**
+             * Replaces each sample alpha value by the maximum representable
+             * alpha value.
+             * @m_since_latest
+             * @see @ref Feature::SampleAlphaToCoverage,
+             *      @ref Feature::SampleCoverage, @ref setSampleCoverage()
+             * @requires_gl Sample alpha to one is not available in OpenGL ES
+             *      and WebGL.
+             */
+            SampleAlphaToOne = GL_SAMPLE_ALPHA_TO_ONE,
+            #endif
+
+            /**
+             * Combines the fragment coverage with the coverage value set in
+             * @ref setSampleCoverage().
+             * @m_since_latest
+             * @see @ref Feature::SampleAlphaToCoverage,
+             *      @ref Feature::SampleAlphaToOne
+             */
+            SampleCoverage = GL_SAMPLE_COVERAGE,
+
             #if !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
             /**
              * Sample shading.
@@ -863,6 +895,15 @@ class MAGNUM_GL_EXPORT Renderer {
         static void setPointSize(Float size);
         #endif
 
+        /**
+         * @brief Set sample coverage parameters
+         * @m_since_latest
+         *
+         * Initial value is @cpp 1.0f @ce and @cpp false @ce.
+         * @see @ref Feature::SampleCoverage, @fn_gl_keyword{SampleCoverage}
+         */
+        static void setSampleCoverage(Float value, bool invert);
+
         #if !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
         /**
          * @brief Set min sample shading value