Support sRGB color space in RenderTexture

When the sf::ContextSettings asks for an sRGB capable buffer, set the
rendered image to sRGB mode and convert pixels when drawing.

This is the choice of simplicity compared to actually support textures
with more color depth.

src/SFML/Graphics/RenderTexture.cpp

@@ -58,6 +58,9 @@ bool RenderTexture::create(unsigned int width, unsigned int height, bool depthBu
 ////////////////////////////////////////////////////////////
 bool RenderTexture::create(unsigned int width, unsigned int height, const ContextSettings& settings)
 {
+    // Set texture to be in sRGB scale if requested
+    m_texture.setSrgb(settings.sRgbCapable);
+
     // Create the texture
     if (!m_texture.create(width, height))
     {

src/SFML/Graphics/RenderTextureImplFBO.cpp

@@ -296,6 +296,7 @@ bool RenderTextureImplFBO::create(unsigned int width, unsigned int height, unsig
 #ifndef SFML_OPENGL_ES
 
             // Create the multisample color buffer
+            bool srgb = settings.sRgbCapable && GLEXT_texture_sRGB;
             GLuint color = 0;
             glCheck(GLEXT_glGenRenderbuffers(1, &color));
             m_colorBuffer = static_cast<unsigned int>(color);
@@ -305,7 +306,7 @@ bool RenderTextureImplFBO::create(unsigned int width, unsigned int height, unsig
                 return false;
             }
             glCheck(GLEXT_glBindRenderbuffer(GLEXT_GL_RENDERBUFFER, m_colorBuffer));
-            glCheck(GLEXT_glRenderbufferStorageMultisample(GLEXT_GL_RENDERBUFFER, settings.antialiasingLevel, GL_RGBA, width, height));
+            glCheck(GLEXT_glRenderbufferStorageMultisample(GLEXT_GL_RENDERBUFFER, settings.antialiasingLevel, srgb ? GL_SRGB8_ALPHA8_EXT : GL_RGBA, width, height));
 
             // Create the multisample depth/stencil buffer if requested
             if (settings.stencilBits)