[core] Move GLFW stencil clip debug to core

This makes the stencil clip debug available to all platforms.

Fixes #4669.

src/mbgl/renderer/painter_debug.cpp

@@ -5,6 +5,9 @@
 #include <mbgl/shader/plain_shader.hpp>
 #include <mbgl/util/string.hpp>
 #include <mbgl/gl/debugging.hpp>
+#include <mbgl/gl/gl.hpp>
+#include <mbgl/gl/gl_values.hpp>
+#include <mbgl/gl/gl_helper.hpp>
 
 using namespace mbgl;
 
@@ -75,3 +78,39 @@ void Painter::renderDebugFrame(const mat4 &matrix) {
     config.lineWidth = 4.0f * frame.pixelRatio;
     MBGL_CHECK_ERROR(glDrawArrays(GL_LINE_STRIP, 0, (GLsizei)tileBorderBuffer.index()));
 }
+
+void Painter::renderClipMasks() {
+    config.stencilTest = GL_FALSE;
+    config.depthTest = GL_FALSE;
+    config.program = 0;
+    config.colorMask = { GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE };
+
+#ifndef GL_ES_VERSION_2_0
+    config.pixelZoom = { 1, 1 };
+    config.rasterPos = {{ -1, -1, 0, 0 }};
+
+    // Read the stencil buffer
+    const auto& fbSize = frame.framebufferSize;
+    auto pixels = std::make_unique<uint8_t[]>(fbSize[0] * fbSize[1]);
+    MBGL_CHECK_ERROR(glReadPixels(
+                0,                // GLint x
+                0,                // GLint y
+                fbSize[0],        // GLsizei width
+                fbSize[1],        // GLsizei height
+                GL_STENCIL_INDEX, // GLenum format
+                GL_UNSIGNED_BYTE, // GLenum type
+                pixels.get()      // GLvoid * data
+                ));
+
+    // Scale the Stencil buffer to cover the entire color space.
+    auto it = pixels.get();
+    auto end = it + fbSize[0] * fbSize[1];
+    const auto factor = 255.0f / *std::max_element(it, end);
+    for (; it != end; ++it) {
+        *it *= factor;
+    }
+
+    MBGL_CHECK_ERROR(glWindowPos2i(0, 0));
+    MBGL_CHECK_ERROR(glDrawPixels(fbSize[0], fbSize[1], GL_LUMINANCE, GL_UNSIGNED_BYTE, pixels.get()));
+#endif // GL_ES_VERSION_2_0
+}