Add new texel buffer type.

GLSL 3+ shaders can read from texel buffers by declaring a uniform samplerBuffer variable and calling texelFetch with a 0-based index, in the shader code. All attributes in a texel buffer's format array must have the same data format (e.g. all 'floatvec4' formats).

Shader:send accepts a texel Buffer for samplerBuffer uniforms.

Added 'texelbuffer' to love.graphics.getSupported (requires GLSL 3+ and desktop OpenGL).
Added 'texelbuffersize' to love.graphics.getSystemLimits. Returns the max number of values in a texel buffer (array length multiplied by the number of attributes declared in the buffer's format array).

src/modules/graphics/opengl/Graphics.h

@@ -147,6 +147,9 @@ private:
 	bool windowHasStencil;
 	GLuint mainVAO;
 
+	// Only needed for buffer types that can be bound to shaders.
+	StrongRef<love::graphics::Buffer> defaultBuffers[BUFFERTYPE_MAX_ENUM];
+
 	// [rendertarget][readable][srgb]
 	OptionalBool supportedFormats[PIXELFORMAT_MAX_ENUM][2][2][2];