ImageData can track whether they're meant to be loaded into a Texture with a non-sRGB format when gamma correct rendering is enabled.

Fixes #1556.

* Add ImageData/CompressedImageData:setLinear and ImageData/CompressedImageData:isLinear.
  The flag is used as a hint when loading a texture from the data to determine if the format should not be treated as sRGB-encoded. The 'linear' flag in newImage overrides this.

* love.graphics.readbackTexture automatically sets the linear flag on ImageData it returns when the texture's format is linear.
  This allows an ImageData generated via readback to be fed back into a new Texture and the format will match the original Canvas.

* Also clean up some image decoding code.

src/modules/graphics/GraphicsReadback.cpp

@@ -81,6 +81,7 @@ GraphicsReadback::GraphicsReadback(Graphics *gfx, ReadbackMethod method, Texture
 	}
 
 	textureFormat = getLinearPixelFormat(texture->getPixelFormat());
+	isFormatLinear = isGammaCorrect() && !isPixelFormatSRGB(texture->getPixelFormat());
 
 	if (!image::ImageData::validPixelFormat(textureFormat))
 	{
@@ -96,7 +97,7 @@ GraphicsReadback::GraphicsReadback(Graphics *gfx, ReadbackMethod method, Texture
 		if (isRT && !caps.features[Graphics::FEATURE_COPY_RENDER_TARGET_TO_BUFFER])
 			throw love::Exception("readbackTextureAsync is not supported on this system.");
 		else if (!isRT && !caps.features[Graphics::FEATURE_COPY_TEXTURE_TO_BUFFER])
-			throw love::Exception("readbackTextureAsync a with non-render-target textures is not supported on this system.");
+			throw love::Exception("readbackTextureAsync with a non-render-target texture is not supported on this system.");
 	}
 	else
 	{
@@ -158,6 +159,7 @@ void *GraphicsReadback::prepareReadbackDest(size_t size)
 				throw love::Exception("The love.image module must be loaded for readbackTexture.");
 
 			imageData.set(module->newImageData(rect.w, rect.h, textureFormat, nullptr), Acquire::NORETAIN);
+			imageData->setLinear(isFormatLinear);
 			return imageData->getData();
 		}
 	}

src/modules/graphics/GraphicsReadback.h

@@ -103,6 +103,7 @@ protected:
 	StrongRef<love::image::ImageData> imageData;
 	Rect rect = {};
 	PixelFormat textureFormat = PIXELFORMAT_UNKNOWN;
+	bool isFormatLinear = false;
 	int imageDataX = 0;
 	int imageDataY = 0;
 

src/modules/graphics/Texture.cpp

@@ -203,7 +203,7 @@ Texture::Texture(Graphics *gfx, const Settings &settings, const Slices *slices)
 		love::image::ImageDataBase *slice = slices->get(0, 0);
 
 		format = slice->getFormat();
-		if (isGammaCorrect() && !settings.linear)
+		if (isGammaCorrect() && !slice->isLinear())
 			format = getSRGBPixelFormat(format);
 
 		pixelWidth = slice->getWidth();
@@ -886,6 +886,7 @@ bool Texture::Slices::validate() const
 	int w = firstdata->getWidth();
 	int h = firstdata->getHeight();
 	PixelFormat format = firstdata->getFormat();
+	bool linear = firstdata->isLinear();
 
 	if (textureType == TEXTURE_CUBE && w != h)
 		throw love::Exception("Cube textures must have equal widths and heights for each cube face.");
@@ -925,6 +926,9 @@ bool Texture::Slices::validate() const
 
 			if (format != slicedata->getFormat())
 				throw love::Exception("All texture slices and mipmaps must have the same pixel format.");
+
+			if (linear != slicedata->isLinear())
+				throw love::Exception("All texture slices and mipmaps must have the same linear setting.");
 		}
 
 		mipw = std::max(mipw / 2, 1);