context: get_capabilities

[jfcameron]

return information like max texture size, max samples, etc.

[jfcameron]

needed for libraries that depend on implementation specific extensions (depth textures, etc) or specific limits (texture sizes, texture units)

[jfcameron]

bumping. important for libraries that extend the features of gdk::graphics, like gdk_text_renderer and the tilerenderer i just started.

[jfcameron]

capabilities struct, getter is const, is constructed at context construction time. name of capabilites shouldnt be exceedingly opengl specific. for example, to check if the depth attachment extension is available, it should be "bool depth_textures_supported" or something like that. "size_t max_texture_uniforms" instead of "max texture units" etc.