Text: avoid assertion failure on ES2 with EXT_texture_rg.

Affects iOS, among others.

src/Magnum/Text/DistanceFieldGlyphCache.cpp

@@ -60,36 +60,34 @@ DistanceFieldGlyphCache::DistanceFieldGlyphCache(const Vector2i& originalSize, c
 }
 
 void DistanceFieldGlyphCache::setImage(const Vector2i& offset, const ImageView2D& image) {
+    GL::Texture2D input;
+    input.setWrapping(GL::SamplerWrapping::ClampToEdge)
+        .setMinificationFilter(GL::SamplerFilter::Linear)
+        .setMagnificationFilter(GL::SamplerFilter::Linear);
+
     const GL::PixelFormat format = GL::pixelFormat(image.format());
     #if !(defined(MAGNUM_TARGET_GLES) && defined(MAGNUM_TARGET_GLES2))
-    const GL::TextureFormat internalFormat = GL::TextureFormat::R8;
     CORRADE_ASSERT(format == GL::PixelFormat::Red,
         "Text::DistanceFieldGlyphCache::setImage(): expected"
         << GL::PixelFormat::Red << "but got" << format, );
+    input.setImage(0, GL::TextureFormat::R8, image);
     #else
-    GL::TextureFormat internalFormat;
     #ifndef MAGNUM_TARGET_WEBGL
     if(GL::Context::current().isExtensionSupported<GL::Extensions::EXT::texture_rg>()) {
-        internalFormat = GL::TextureFormat::Red;
-        CORRADE_ASSERT(format == GL::PixelFormat::Red,
+        CORRADE_ASSERT(format == GL::PixelFormat::Red || format == GL::PixelFormat::Luminance,
             "Text::DistanceFieldGlyphCache::setImage(): expected"
             << GL::PixelFormat::Red << "but got" << format, );
+        input.setImage(0, GL::TextureFormat::Red, ImageView2D{image.storage(), GL::PixelFormat::Red, GL::PixelType::UnsignedByte, image.size(), image.data()});
     } else
     #endif
     {
-        internalFormat = GL::TextureFormat::Luminance;
         CORRADE_ASSERT(format == GL::PixelFormat::Luminance,
             "Text::DistanceFieldGlyphCache::setImage(): expected"
             << GL::PixelFormat::Luminance << "but got" << format, );
+        input.setImage(0, GL::TextureFormat::Luminance, image);
     }
     #endif
 
-    GL::Texture2D input;
-    input.setWrapping(GL::SamplerWrapping::ClampToEdge)
-        .setMinificationFilter(GL::SamplerFilter::Linear)
-        .setMagnificationFilter(GL::SamplerFilter::Linear)
-        .setImage(0, internalFormat, image);
-
     /* Create distance field from input texture */
     TextureTools::distanceField(input, texture(), Range2Di::fromSize(offset*scale, image.size()*scale), radius, image.size());
 }