Add love.graphics.getQuadIndexBuffer.

love's pre-generated quad index buffer is useful when combined with drawShaderVertices. Note that it uses 16 bit indices so it can only draw up to 16k quads in a single call.

src/modules/graphics/opengl/Buffer.cpp

@@ -166,7 +166,7 @@ bool Buffer::supportsOrphan() const
 
 void *Buffer::map(MapType /*map*/, size_t offset, size_t size)
 {
-	if (size == 0)
+	if (size == 0 || isImmutable())
 		return nullptr;
 
 	Range r(offset, size);
@@ -229,7 +229,7 @@ void Buffer::unmap(size_t usedoffset, size_t usedsize)
 
 void Buffer::fill(size_t offset, size_t size, const void *data)
 {
-	if (size == 0)
+	if (size == 0 || isImmutable())
 		return;
 
 	size_t buffersize = getSize();