svpaint: Change a variable from global to local

This fixes a warning from LGTM:

    Poor global variable name 'rgb'. Prefer longer, descriptive
    names for globals (eg. kMyGlobalConstant, not foo).

Signed-off-by: Stefan Weil <sw@weilnetz.de>

src/viewer/svpaint.cpp

@@ -32,7 +32,7 @@
 #include <iostream>
 
 // The current color values we use, initially white (== ScrollView::WHITE).
-int rgb[3] = { 255, 255, 255 };
+static int rgb[3] = { 255, 255, 255 };
 
 class SVPaint : public SVEventHandler {
  public: