Added sRGB (gamma-correct) support for Images, Canvases, and the main screen.

love.graphics.newImage(path, “srgb”) creates a new Image whose texels are treated as being in the sRGB color space, so they are linearized when drawing/sampling from the image.

love.graphics.newCanvas(w, h, “srgb”) creates a new Canvas whose texels are treated as being in the sRGB color space, so drawing to the Canvas does a linear->sRGB conversion (but blends linearly), and sampling from it (drawing it or using it in a shader) converts from sRGB to linear space.

The "srgb" window flag does the same as Canvases for the main screen.

src/modules/graphics/Graphics.cpp

@@ -186,6 +186,7 @@ StringMap<Graphics::Support, Graphics::SUPPORT_MAX_ENUM>::Entry Graphics::suppor
 	{ "dxt", Graphics::SUPPORT_DXT },
 	{ "bc5", Graphics::SUPPORT_BC5 },
 	{ "instancing", Graphics::SUPPORT_INSTANCING },
+	{ "srgb", Graphics::SUPPORT_SRGB },
 };
 
 StringMap<Graphics::Support, Graphics::SUPPORT_MAX_ENUM> Graphics::support(Graphics::supportEntries, sizeof(Graphics::supportEntries));