Add sync and async texture and buffer readback APIs.

- Add imagedata = love.graphics.readbackTexture(texture, [slice, mipmap, x, y, w, h, [dest, destx, desty]]).
- Add readback = love.graphics.readbackTextureAsync(texture, [slice, mipmap, x, y, w, h, [dest, destx, desty]]).
- Add bytedata = love.graphics.readbackBuffer(buffer, [offset, size, [dest, destoffset]]).
- Add readback = ove.graphics.readbackBufferAsync(buffer, [offset, size, [dest, destoffset]]).
- Deprecate Texture:newImageData.

The async variants return a new GraphicsReadback object. It has the following methods:
- isComplete()
- hasError()
- wait()
- getBufferData()
- getImageData()
- update() (called automatically every frame by love, not normally needed).

Support notes:
- readbackBuffer and readbackBufferAsync require buffer copying support.
- readbackTexture with a render target (canvas) is always supported. readbackTexture with a non-render-target requires copy-texture-to-buffer support.
- readbackTextureAsync with a render target requires copy-render-target-to-buffer support. readbackTextureAsync with a non-render-target requires copy-texture-to-buffer support.

src/modules/graphics/wrap_Texture.cpp

@@ -385,16 +385,15 @@ int w_Texture_replacePixels(lua_State *L)
 
 int w_Texture_newImageData(lua_State *L)
 {
+	luax_markdeprecated(L, 1, "Texture:newImageData", API_METHOD, DEPRECATED_RENAMED, "love.graphics.readbackTexture");
+
 	Texture *t = luax_checktexture(L, 1);
-	love::image::Image *image = luax_getmodule<love::image::Image>(L, love::image::Image::type);
 
 	int slice = 0;
-	int mipmap = 0;
-
 	if (t->getTextureType() != TEXTURE_2D)
 		slice = (int) luaL_checkinteger(L, 2) - 1;
 
-	mipmap = (int) luaL_optinteger(L, 3, 1) - 1;
+	int mipmap = (int) luaL_optinteger(L, 3, 1) - 1;
 
 	Rect rect = {0, 0, t->getPixelWidth(mipmap), t->getPixelHeight(mipmap)};
 	if (!lua_isnoneornil(L, 4))
@@ -405,8 +404,12 @@ int w_Texture_newImageData(lua_State *L)
 		rect.h = (int) luaL_checkinteger(L, 7);
 	}
 
+	auto gfx = Module::getInstance<Graphics>(Module::M_GRAPHICS);
+	if (gfx == nullptr)
+		return luaL_error(L, "Cannot find Graphics module.");
+
 	love::image::ImageData *img = nullptr;
-	luax_catchexcept(L, [&](){ img = t->newImageData(image, slice, mipmap, rect); });
+	luax_catchexcept(L, [&](){ img = gfx->readbackTexture(t, slice, mipmap, rect, nullptr, 0, 0); });
 
 	luax_pushtype(L, img);
 	img->release();
@@ -501,8 +504,11 @@ const luaL_Reg w_Texture_functions[] =
 	{ "setDepthSampleMode", w_Texture_setDepthSampleMode },
 	{ "generateMipmaps", w_Texture_generateMipmaps },
 	{ "replacePixels", w_Texture_replacePixels },
-	{ "newImageData", w_Texture_newImageData },
 	{ "renderTo", w_Texture_renderTo },
+
+	// Deprecated
+	{ "newImageData", w_Texture_newImageData },
+
 	{ 0, 0 }
 };