Add buffer stats to love.graphics.getStats.

- Add 'buffers' field, to count the total number of graphics buffer objects.
- Add 'buffermemory' field, to estimate the total number of bytes of graphics memory used by all buffers.

src/modules/graphics/Buffer.cpp

@@ -29,6 +29,9 @@ namespace graphics
 
 love::Type Buffer::type("GraphicsBuffer", &Object::type);
 
+int Buffer::bufferCount = 0;
+int64 Buffer::totalGraphicsMemory = 0;
+
 Buffer::Buffer(Graphics *gfx, const Settings &settings, const std::vector<DataDeclaration> &bufferformat, size_t size, size_t arraylength)
 	: arrayLength(0)
 	, arrayStride(0)
@@ -240,10 +243,15 @@ Buffer::Buffer(Graphics *gfx, const Settings &settings, const std::vector<DataDe
 	if (texelbuffer && arraylength * dataMembers.size() > caps.limits[Graphics::LIMIT_TEXEL_BUFFER_SIZE])
 		throw love::Exception("Cannot create texel buffer: total number of values in the buffer (%d * %d) is too large for this system (maximum %d).",
 			(int) dataMembers.size(), (int) arraylength, caps.limits[Graphics::LIMIT_TEXEL_BUFFER_SIZE]);
+
+	++bufferCount;
+	totalGraphicsMemory += size;
 }
 
 Buffer::~Buffer()
 {
+	totalGraphicsMemory -= size;
+	--bufferCount;
 }
 
 int Buffer::getDataMemberIndex(const std::string &name) const