OGL: Use floating points for viewport parameters.

As told by neobain, ARB_viewport_array introduced float viewports.
They must be supported on OGL4.1+, but it's usually also implemented on 3.3 GPUs.

Source/Core/VideoBackends/OGL/Src/Render.h

@@ -27,6 +27,7 @@ extern struct VideoConfig {
 	bool bSupportSampleShading;
 	GLSL_VERSION eSupportedGLSLVersion;
 	bool bSupportOGL31;
+	bool bSupportViewportFloat;
 	
 	const char *gl_vendor;
 	const char *gl_renderer;