BaseContext: optimize getGradientTexture

// TODO: need to dispose these when the context is disposed...
    private final Map<FontStrike, GlyphCache>
        greyGlyphCaches = new HashMap<FontStrike, GlyphCache>();
    private final Map<FontStrike, GlyphCache>
        lcdGlyphCaches = new HashMap<FontStrike, GlyphCache>();
    GlyphCache[] recentCaches = new GlyphCache[8];

getGradientTexture():
could save some work if we converted the *GradientContext classes to produce ByteRgbaPre data instead of IntArgbPre data...