Fix GL version check for arbitrary texture-to-buffer copy support.

love never creates a GL4.5 context, so we also need to check for the extension that provides the same functionality.

src/modules/graphics/opengl/OpenGL.cpp

@@ -1541,7 +1541,7 @@ bool OpenGL::isCopyBufferToTextureSupported() const
 bool OpenGL::isCopyTextureToBufferSupported() const
 {
 	// Requires glGetTextureSubImage support.
-	return GLAD_VERSION_4_5;
+	return GLAD_VERSION_4_5 || GLAD_ARB_get_texture_sub_image;
 }
 
 bool OpenGL::isCopyRenderTargetToBufferSupported() const

src/modules/graphics/opengl/Texture.cpp

@@ -563,7 +563,7 @@ void Texture::copyToBuffer(love::graphics::Buffer *dest, int slice, int mipmap,
 	// treating the pointer as a byte offset.
 	uint8 *byteoffset = (uint8 *)(ptrdiff_t)destoffset;
 
-	if (GLAD_VERSION_4_5)
+	if (gl.isCopyTextureToBufferSupported())
 	{
 		if (isCompressed())
 			glGetCompressedTextureSubImage(texture, mipmap, rect.x, rect.y, slice, rect.w, rect.h, 1, size, byteoffset);