Text: fall back to full RGBA on WebGL 1 or ES2 w/o EXT_texture_rg.

RGB was a hopeful intention that never really worked in practice. Or, if
it worked, it couldn't be queried back, and the driver used RGBA
internally anyway.

src/Magnum/Text/DistanceFieldGlyphCache.cpp

@@ -43,11 +43,13 @@ DistanceFieldGlyphCache::DistanceFieldGlyphCache(const Vector2i& size, const Vec
     #if !(defined(MAGNUM_TARGET_GLES) && defined(MAGNUM_TARGET_GLES2))
     GlyphCache(GL::TextureFormat::R8, size, processedSize, Vector2i(radius)),
     #elif !defined(MAGNUM_TARGET_WEBGL)
-    /* Luminance is not renderable in most cases */
+    /* Luminance is not renderable in most cases. RGB is *theoretically*
+       space-efficient but practically the driver uses RGBA internally anyway,
+       so just use RGBA. */
     GlyphCache(GL::Context::current().isExtensionSupported<GL::Extensions::EXT::texture_rg>() ?
-        GL::TextureFormat::R8 : GL::TextureFormat::RGB8, size, processedSize, Vector2i(radius)),
+        GL::TextureFormat::R8 : GL::TextureFormat::RGBA8, size, processedSize, Vector2i(radius)),
     #else
-    GlyphCache(GL::TextureFormat::RGB, size, processedSize, Vector2i(radius)),
+    GlyphCache(GL::TextureFormat::RGBA, size, processedSize, Vector2i(radius)),
     #endif
     _distanceField{radius}
 {
@@ -65,7 +67,7 @@ DistanceFieldGlyphCache::DistanceFieldGlyphCache(const Vector2i& size, const Vec
     #if defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
     /* Luminance is not renderable in most cases */
     if(!GL::Context::current().isExtensionSupported<GL::Extensions::EXT::texture_rg>())
-        Warning() << "Text::DistanceFieldGlyphCache:" << GL::Extensions::EXT::texture_rg::string() << "not supported, using inefficient RGB format for glyph cache texture";
+        Warning() << "Text::DistanceFieldGlyphCache:" << GL::Extensions::EXT::texture_rg::string() << "not supported, using a full RGBA format for the distance field texture";
     #endif
 }
 

src/Magnum/Text/Test/DistanceFieldGlyphCacheGLTest.cpp

@@ -258,8 +258,8 @@ void DistanceFieldGlyphCacheGLTest::setProcessedImage() {
 
     #ifdef MAGNUM_TARGET_GLES2
     /* Ugh, don't want to bother implementing this */
-    if(cache.processedFormat() == PixelFormat::RGB8Unorm)
-        CORRADE_SKIP("A three-component input is expected on ES2, skipping due to developer laziness.");
+    if(cache.processedFormat() == PixelFormat::RGBA8Unorm)
+        CORRADE_SKIP("A four-component input is expected on ES2, skipping due to developer laziness.");
     #endif
 
     /* Clear the texture first, as it'd have random garbage otherwise */