GL: add some other rather crappy ways to detect ANGLE.

Not proud-- actually, WHAT A TRASH FIRE THIS WEBDEV IS FFS

src/Magnum/GL/Context.h

@@ -430,8 +430,13 @@ class MAGNUM_GL_EXPORT Context {
              * browsers on Windows for WebGL. See also
              * @ref DetectedDriver::SwiftShader. On WebGL, if
              * @webgl_extension{WEBGL,debug_renderer_info} is not available,
-             * the detection is done by checking if the line size range is just
-             * 1, which is always the case on D3D but not always on GL.
+             * the detection is attempted by checking for a presence of
+             * @webgl_extension{ANGLE,instanced_arrays} on WebGL 1 (which may
+             * also be implemented by other drivers than ANGLE) or
+             * @webgl_extension{WEBGL,multi_draw} on WebGL 2 (which may not be
+             * available yet on older ANGLE versions), and as a fallback by
+             * checking if the line size range is just 1, which is always the
+             * case on D3D but not always on GL.
              * @requires_gles ANGLE doesn't support desktop OpenGL contexts.
              */
             Angle = 1 << 1,

src/Magnum/GL/Implementation/driverSpecific.cpp

@@ -487,6 +487,23 @@ auto Context::detectedDriver() -> DetectedDrivers {
     if(renderer.contains("ANGLE"_s))
         *_detectedDrivers |= DetectedDriver::Angle;
     #ifdef MAGNUM_TARGET_WEBGL
+    /* Otherwise assume ANGLE is present if WEBGL_debug_renderer_info isn't
+       present (which would tell us so already) and the ANGLE_instanced_arrays
+       is present on WebGL 1. Although e.g. Firefox exposes it even though it
+       renders directly through GL drivers on Linux, so this may catch more
+       drivers than just ANGLE. */
+    /** @todo this (and below) incorrectly detects ANGLE on FF 92+, as it has
+        the WEBGL_debug_renderer_info disabled */
+    #ifdef MAGNUM_TARGET_GLES2
+    else if(!isExtensionSupported<Extensions::WEBGL::debug_renderer_info>() && isExtensionSupported<Extensions::ANGLE::instanced_arrays>())
+    /* Or if WEBGL_debug_renderer_info isn't present and WEBGL_multi_draw
+       (which is based on ANGLE_multi_draw) is present on WebGL 2. This
+       extension is rather recent (appearing in browsers in late 2020) so it
+       may not catch all ANGLE implementations. */
+    #else
+    else if(!isExtensionSupported<Extensions::WEBGL::debug_renderer_info>() && isExtensionSupported<Extensions::WEBGL::multi_draw>())
+    #endif
+        *_detectedDrivers |= DetectedDriver::Angle;
     /* Otherwise try to detect a D3D ANGLE backend by querying line width. It's
        always exactly just 1 on D3D, usually more on GL, not sure about Metal.
        so this is not a 100% match. Sources: http://stackoverflow.com/a/20149090